Without a box label for a specific cartridge, I know of no way to accurately determine the age of specific 8 mm Nambu Cartridges. All of the Japanese-made 8 mm Nambu rounds I have encountered, made in 1945 and before, are without headstamp, although there are some headstamped post-war Japanese-made rounds with headstamps, and one exception in pre-1946 Japanese round, a wood-bullet grenade launching cartridge. The latter has been normally found with a red wood bullet, although at least one with a plain wood bullet is known. These have a single character, composed of a curved line with either one or two “tick marks” above the curve. They were used, evidently primarily by police, to launch tear-gas grenades. We assume it was a cup-type launching apparatus and existed for the Type 14 Japanese Pistol, as well as for the earlier Type 26 (1893) 9 mm Japanese revolver.
This cartridge was made for at least 43 years in Japan, and perhaps a few years longer. This does not include the post-war production by Asahi Okuma. The first pistol for it was the original “Type Nambu” pistol, called by the slang name of “Grandpa” or “Grandpop” Nambu, which was introduced, but not formally adopted, in 1902. The length of time this cartridge was manufactured, at the 1st and 2nd Tokyo Artillery Arsenals, guarantees variations. I have 18 variations of the ball cartridge, differing primarily in the size, shape and placement of the stab-type bullet crimps in the neck of the cartridges, at least two different bullet jacket materials, gilding metal (copper) and cupro-nickel, and some minor differences in the width of the extractor groove and extractor-groove bevels. Also found are cartridges with no stab-type bullet crimps. This occurs in both ball rounds and dummy cartridges. (I have five variations of the Pre-1946 dummy cartridge, for example.)
It is an interesting cartridge, that may well predate the 20th Century, as development of this round by Kijiro Nambu undoubtedly began as part of the Type 30 (30th year of Meiji, 1897 on our calender) Automatic Pistol Plan.
The gilding metal bullet, by the way, was a late-comer on the scene, with manufacture beginning c.1942.
hope this helps.
Edited only to correct an improper word form.
Reference: collection at hand; personal file notes; “Japanese Military Cartridge Handguns 1893-1945,” by Harry L. Derby III & James D. Brown, published 2003, pages 300-305.