Do you have any commercially-made Arisaka ammo to compare it to? sounds like these are old gov't 30-06 cases that were trimmed down to 7.7 x 58 Japanese. You can measure the specs of the cases to be sure.
Anyway, are you going to reload them?
I've got a bunch of 7.7 Arisaka brass, but have not bought reloading equipment for it. Probably will not reload it; at least not this lifetime because I have so much other stuff to load. Commercial ammo is available from Norma (very expensive, and from what I hear it is not the best anyway). Also, Hornady loads 7.7 Japanese and sells it, but I am not sure if they use once-fired cases, and anyway with the Great Ammo Panic still going strong I am sure that Hornady has other things to do with their presses than make a small niche cartridge. When Hornady is loading it, you can get it direct from Graf's and Graf's also wholesales it to Cabela's to sell retail. Used to be kind of a seasonal run that showed up around April or May and took a few months to sell out. Now, there is no telling when they'll make it again. I am not positive, but I think Graf's collects used brass and sends it to be filled. I know they have done that with a commercial reloader called Precision Cartridge Co. Not sure where Hornady gets their 7.7 brass. Anyway, just wondering if you're going to load that brass you have. There are other sources for reloaded 7.7 out there but I can't vouch for them.