The Japanese military made the decision to move from a 6.5mm infantry rifle to a more powerful 7.7mm cartridge in the 1930s, with specifications for the new rifle proposed in 1938. In response, the Nagoya and Kokura Arsenals developed new models of rifle and carbine.
What we are looking at today is one of the Nagoya (Plan #1) experimental carbines; serial number 4 to be specific. It has a variety of novel features, including:
Spring loaded buttplate for recoil reduction
Unique muzzle brake
New aperture rear sight
Threaded cleaning rod
New magazine retention system
Recoil bolt in stock
These carbine, and the other experimental patterns, were tested at the Futsu Proving Ground in 1939. Elements of both designs were ultimately used to create the Type 99 adopted later in 1939 (Japanese calendar year 2599).