WARNING Waffle but of great interest if you want to understand what is known about where “Bengalese” originated.
I guess before we jump into anything it would be a good idea to discover the origins of our birds.
In many publications and in conversation with other Bengalese Finch fanciers I have been surprised to find that many still adhere to the belief that the Bengalese Finch is the result of hybridisation with other species of lonchura. Here is much of the information I have gleaned from publications and conversation, along with some speculation on my part on the History and origins of the Bengalese Finch.
The birds we term "Continental Bengalese" that originate from The Netherlands and Belgium are well documented as being the result of the introduction of other species, notably the Black and the White-headed Nuns, L.atricapilla and L.maja respectively. But this is recent history and does not concern us here in the origins of the Bengalese.
In Dr Arthur Butler's, Foreign Finches in Captivity, published in 1894 we can find the first speculation on this hybrid origin.In this book is F.W. Frohawk's colour plate "The Bengalee" below which is stated through the Latin names, that this is a Silverbill x Striated Finch hybrid. In the accompanying text Dr Butler explains that "this delightful little fertile hybrid was produced by the Japanese, who carefully bred and selected for some hundreds of years, until they developed the three well known forms. The white pied with brown, which is slightly the largest, and has a dark upper mandible, the white pied with fawn, which has flesh tinted beak and legs, and the white type which is the smallest".
Dr Butler had at first thought the Bengalee to be a domesticated Sharp Tailed Finch, because of their similarity in song, but he had "yielded to Mr Joseph Abraham's far wider experience, when he wrote that the Bengalee is a cross between the Striated Finch and the Indian Silverbill”. This I believe is the source from which the misconception first arose concerning the hybrid origins of the Bengalese.  

Robin Restall in my opinion the most knowledgeably authority on the lonchura family, tells us in his excellent book Munias & Mannikins, that "This is a domesticated form of the Striated Muinia, almost certainly the Chinese race, L.s. swinhoei"
Dr Desmond Morris compared behaviour of the Bengalese with that of the Striated Finch, and concluded that the behaviour patterns of the two species were the same, while the Bengalese had no behaviour components of any other munia species he could obtain, in particular the Silverbill.
Erica Elsner also worked with a large breeding stock of Bengalese at Oxford, and published one of the most definitive studies on the Bengalese ever written, in the May/June 1957 issue of The Avicultural Magazine. By comparing the near Self and Self Bengalese produced in the breeding experiment with an extensive range of skins of all the geographical races of the striated finch Lonchura Striata acuticauda, she concluded that the Bengalese is a domestic form of the geographical race of the Sharp Tailed Finch inhabiting South Eastern China (Lonchura Striata squamicollis) and there is no reason to suppose it is a hybrid with other species.
If you have ever kept White Rumped Munias, Lonchura striata, you would after a short time have no doubt that this in it's pure form is the bird from which our Bengalese originated, and there is no hybrid blood in the Bengalese.
I also began to wonder about accounts that the Bengalese was developed in Japan from wild birds imported from China.
The Japanese aviculturist, Prince Nobusuke Taka-Tsukasa of Tokyo states in his article published in the Avicultural Magazine, February 1922, that "an old Japanese book on cage birds tells us that this bird comes from China, and in the original species the upper parts are dark brown, and the lower white with dusky streaks, it's bill is black and it's legs are pale bluish grey, and it's tail long and pointed”. He further stated “According to this description I think the Bengalese comes from the Sharp tailed finch, which is common in the southern part of China & Formosa. This bird was imported into Japan about 200 years ago, and during 50 years it was distributed in all parts of Japan, but still during this time (the first 50 years) it seems that the white & cinnamon varieties did not appear".
This as far as I can find out this is the only published account of the Japanese history of the Bengalese, the only other one by Kozo Nakunishi published in 1929 follows this account and time frame almost exactly, so is probably taken from this article by Prince Nobusuke, and who would contradict a Japanese prince?  Still it appears to me the whole Bengalese history in Japan relies on the Prince's "old Japanese book on cage birds" of which there is no information given.
I don't for one second deny the amazing accomplishments and contributions made by the Japanese fanciers to the development of the Bengalese, but I begin to wonder about this idea that the Japanese were the creators/breeders of the first mutation Sharp tailed munias.
Today we get many species imported from China, and Lonchura Striata, when I ask about obtaining them at the importers, I am told just "little brown birds" which are of no value or interest to most fanciers and hardly worth importing. They don't have any song to speak of etc, etc, and today I imagine they could be here in a day straight from China. Why would the Japanese in the 1600s with the journey they had to undertake to China, return with these "little brown birds" and apparently in enough quantity for them to be "during 50 years distributed to all parts of Japan". I am told that today and throughout Chinese history they are called "The Emperors Birds" and where kept by the Emperors in China as something special, so what could this be? Could  it be because they were already Pied and pure White?  Could it be the reason the Japanese bought them home, when there were so many other exotic, colourful and melodious Chinese species they could have returned with. Let us remember the Japanese were traders, and looking to turn a profit, exactly like the bird traders today, who say, "who wants just little brown birds".
Attempts have been made to discover some information on the Chinese history of cage birds from the National Library of China, but I have been told that just to be informed if they had any references to "Bengalese" would cost  $250 US, a bit over the top just for that information I thought.
A friend Jenn-Ching Luo from New York who is also very interested in discovering the origins of the Bengalese tells me.
"Since the 19th century, there were no Chinese aviculturists (China was in trouble that time, and then Chinese communist had Cultural Revolution. Those killed (I think he means discouraged or banned bird keeping here) most Chinese aviculturists.). There are some problems to trace it back." 
He tells me concerning his research in China and his contact there, "That gentleman, who is only a bird keeper (not an expert), will keep trying to find more ancient books, and will let me know if he finds anything. He cannot understand what kind of theory to support that Japanese developed the original mutations. The years 1620s mentioned in article about history of Bengalese, (the Avicultural Magazine, February 1922) seems not correct, either. History book shows ancient Japan and Mine Dynasty did not have a good relation around 1620s.
He did predict "the birds might be introduced to Japan earlier", possibly in T'ang Dynasty (618-907). Japan sent many experts to T'ang Dynasty to learn what Japanese were interested in, and Japan imported and brought many things back to Japan, including fruit tree, rice, vegetable (possible seeds only), poultry, livestock, culture, and so on. Japan also modified the traditional Chinese characters into Japanese in the T'ang Dynasty. Ancient book shows pet birds were bred in cages in T'ang
Dynasty, very early (The problem is which kind of pet birds. No details in the book. Bengalese or not? Unclear.). T'ang was a great time in Chinese history. He thinks those "birds" might be introduced to Japan in T'ang Dynasty. No further proof, prediction only. It may be very difficult to find out the real history."
Further to this a Chinese contact Gaowei sent me the following information:
I asked my friend to check for some the published information in China that concerned the lonchura striata; unfortunately, the books are all in Chinese. so I have translated the main history for you.  The ancestors of lonchura striata is found in North Vietnam 4000 years ago. at that time North Vietnam used to be a province of China.  and about 3500 years ago, people had begun to keep them in their homes in the area of North and South Vietnam. and at that time, the bird was called Pumped Munia and Bengalese. More and more people became interested in the birds, the domesticus bird became popular in China in the Ming and Qing dynasty, and the bird was called sisters owing to their beautiful appearance. About 120 years ago, Japanese began to breed the bird and made a mixture with a kind of domestic bird of Japan, that is why all the published information said that the bird is first bred in Japan instead of China. But actually, the bird is very popular in China and Chinese also developed the different branches of their family. which have the different appearances.
What I deduce from this is, unless these birds were already colour variants, i.e. variegated or whites I find it difficult to believe they would be described as being called "sisters, because of their beautiful appearance" And also surprised to read that the Japanese "made a mixture with a kind of domestic bird of Japan" and do not believe this to be correct.
I have no doubt that the Bengalese is the domesticated form of Lonchura striata, possibly produced from the crossing of various geographical races of that spec