I was pretty sure myself that 30-06 was 7.62x63 also...   7.7 Jap is actually 7.7x58.  Could this be a cartridge that was chambered to feed in a Type 99?   The case dimensions for 7.7x58 and US M2 Ball (30-06 or 7.62x63) are very similar... and before Norma started mass production of 7.7 jap ammo, most Arisaka shooters made cases by trimming and re-sizing 30-06 cases.  Also, .30 caliber jacketed bullets are typically .308 diameter, while 7.7 (and brittish .303) bullets are .312 diameter, so technically, a .30 caliber bullet could be fired in a Type 99 Arisaka if the case dimensions were correct.  Food for thought????