I propose that either they were all built that way or none were built that way. The war expediency doesn't cut it because they did not start producing simpler rifles until late in war and they were simply simpler construction, elimination of not needed parts, etc. Only at the very, very end were they putting together rifles that might be unsafe.
All that being said, I think the first thing to be eliminated would be a Type 2 paratrooper rifle. They sure weren't conducting airborne operations late war and they would concentrate on the simpler straight design.
So in my mind, that eliminates the late war expediency argument.
As for them all being built that way, I just don't see it.
And yes, all "kind" of a guess but when we can look at a schematic which shows it as one unit, I think that's pretty strong evidence.