There can be more than 1 prototype

It's unlike any standard, and apparently the book that scn.ru scanned it from says it's a prototype. This is one of the entries that actually has a reference in it, that Larry already posted:
"Ki-67 Prototype
Unit: Rikugun Koku Shinsa-bu (Army Aviation Experimental Department)
Serial: 2
Ki-67 Prototype No.2 at Kagamigahara Army Air Base, Gifu Prefecture, February 1943.
Artist: © Rikyu Watanabe
Source: 'Army type 4 Heavy Bomber Hiryu (Ki-67 Peggy)', Famous Airplanes of the World No.98, Bunrindo Co."
EDIT: P.S. I bet the one behind it is prototype No. 1, in the photo!