EDIT: In response to WMaker:
First, try a search anywhere for a "model 12" -- it doesn't exist. Then by searching more the best explanation presents itself and the other sources support it.
In short, I scoured the web all day with the curiousity of "what is a model 12" -- only to find there wasn't one. That is, I scoured it at work between doing stuff. Checked about 20 different links and resources online and even read some comments about Yamamoto's G4M1.
Here's the one that supports pretty much what I figured out when doing my searches:
http://www.j-aircraft.com/faq/G4M.htm#G4M1 Type 1 Tail Gun QuestionYou'll note Jim Long is the one answering the question. He seems to be somewhat known (I hadn't heard about him before I started looking into the Ki-61 and Ki-100 some more in the past month, but then I don't normally research the Japanese craft, which are more of his specialty). He translates primary japanese documents, has an archive of US intelligence reports from WW2, is a modeler and IPMS member. He's been published on at least a handful of things including some books and some IPMS articles.
He's a bit of an expert when it comes to the finer details of Ki-61 performance and history, as well. Likes to discuss a lot of his work and findings online at j-aircraft.