Since Japanese made ammunition has shown up in other calibers (6.35mmB, 7.65mmB, etc) I have long suspected that Japan made 9imm Para before or during WWII. For many years Woodin Lab had a unheadstamped case that was reported to be Japanese, but a few years ago when Ken Elks was working on his book, he and Bill concluded that it was not Japanese. I have found no credible evidence of Japanese manufacture of 9mm Para before the post-WWII era.
Interestingly, the same is true of China, pre-1945. Lots of production of 7.63mm Mauser and also production of 7.65mmB and revolver cartridges. Records of production of 6.35mmB but no evidence of any 9mm Para production. The Woodin Lab has a single round that may be an early Chinese 9mmP but the headstamp looks to me as if it were early Thai rather than Chinese.
If someone has evidence on either country making 9mmP before the end of WWII, Please share it.
Fede, Thanks for the photo of the pistol. I have never seen or heard of a 9mmPara Nambu before. This may also be a product of Vietnam. During their long series of wars, they converted quite a few pistols to 9mmP. I have seen 10 or 12 Chinese pistols originally chambered for 7.63mm Mauser that have been modified to fire 9mm Para. As JohnS says, it depends on what ammo is available. I once saw a French MAT machine pistol that was either made for or modified for a 7.5mm rimfire cartridge-apparently somebody had a bunch of it and made/modified a gun to fire it.