Hi Charley,
That's certainly an interesting and odd looking projectile you have there!
As for your assumption that it may be Japanese, I have to say I have some difficulties with that. There are similarities, but there are also a lot of things that make me think it's something else.
The similarities:
-The overall shape, with a bourelet and thin single driving band looks reminiscent of IJA projectiles.
-The 'plug' as you call it, may actually be a very rudimentary fuse, being the same style fuse as is found on several of the smaller calibre IJA Ma 'Special Incendiaries': they were basically RDX loaded, having a PETN filled brass cap that served as fuse. This looks highly similar to the one shown in your pictures.
The mismatches:
-You quote a projectile body length of 176mm, whereas all IJA 75mm projectiles seem to be much taller. The ones I can find the dimensions of all seem to have a body length (fuseless) of at least 240mm.
-The absence of markings is odd. The markings (if that's what they are) on the base do not look Japanese, and normally Japanese projectiles, or maybe better put: Showa era Japanese projectiles, did have markings stamped into them. Same for the fuse. The fuse I have on a 20x95 Ho-5 Ma 202 special incendiary round, features the Tokyo arsenal mark, and that's a tiny fuse compared to this one...
-The base of the projectile looks very straightish for a Japanese projectile; most of them have more rounded or even boat tailed bases.
-The thin drive band rules out any Showa era IJN projectiles, which all used far wider driving bands, and the overall shapes of the 3" IJN projectiles is very different too.
-I cannot find any references to large 'PETN fuses' in OpNav 1667, neither in the IJA section, nor in the IJN section.
So, so much for Showa era Japanese projectiles, but... I suppose there is a chance of course that it is an older type of Japanese projectile. If indeed the cap is a PETN fuse, the projectile would really have to be of the HE(I) type...
Anyone any ideas?
Cheers,
Olafo
