Desparado - it is my opinion that most collectors who deal at all with the Nambu Pistol calibers, 7 mm or 8 mm, could tell when an 8 mm cartridge is Japanese. Prior to 1946, I know of no absolutely confirmed production of this caliber outside of Japan. There are rumors of ammunition made in Thailand, which certainly is not impossible, but I don’t recall ever seeing any Thai packaging for it. Thailand did use some Nambu pistols, and they are occasionally found with Thai-language characters on the grips back-strap.
There are many variations found within the Japanese-made 8 mm Nambu cartridges, but then after all, the cartridge dates to right around the turn of the century in 1900. The first Nambu 8 mm Pistol was brought out in 1902, and called simply the “Type Nambu,” but is referred to today by collectors as the “Grandpa Nambu.” Of course, there is always a developmental period prior to the serial production of any firearm, so it is possible the 8 mm cartridge was designed and made, albeit in small quantities, earlier than 1902. I know of no actual documentation for this, however. This caliber was made, therefore, for at least 43 years (pre-1946) and likely by more than one factory, so one could readily expect variations in the cartridges.
However, Japanese 8 mm Nambu cartridges have their own “look” which is difficult to define here. I will have to let that rest with my earlier comment that most who seriously collect Japanese ammunition can spot the Japanese versions immediately. After 1906, some 8 mm Nambu cartridges were made in Japan, but they are headstamped. Since WWII, there have been many companies that have made this caliber, primarily using cartridge cases of other calibers, reformed to the 8 mm Nambu. I have about 40 of these variations in my own collection. Further, in fairly recent years, many companies have made this cartridge case properly headstamped, so ammunition could be made available for the souvenir pistols that folks wanted to shoot. They are a subject of their own.