Both could be correct: The t89 Vickers fixed Aircraft and the T92 Naval (Vickers)Fixed Aircraft were essentailly the Same gun, only different Cartridges (same (7,7mm ) calibre.
The t89 Army Airforce used the type 89 7,7x58SR (predecessor of the Army T92(Nambu) Cartridge, and the Navy Gun used the 7,7x56R (aka.303) Naval MG cartridge ( same as the Naval Lewis.)
The two types of Vickers gun Clones were Copied (with slight changes,so are NOT interchangeable with the original Vickers Aircraft Gun) in the 1920s. The Navy retained the original (.303) cartridge, whilst the Army Airforce developed the Semi-rimmed T89 and T92 cartridges ( eventually the Type 89 cartridge was supplanted by the T92 (heavier ball, and load), and ammo packets of T92 ammo for use in Airforce guns was marked "For use in both T89 Airforce and T92 Army Guns".
You guys are good! I think it's a Type 89 7.7mm machine gun judging by the vent hole pattern on the barrel. We found at least 3 types on Peleliu that the Japanese salvaged out of their demolished aircraft to beef-up the defenses in the ridge system. This gun was found near the entrance to a cave.