Neither "T" nor "1" marks..they are a Japaneseicon Kana Mark, usually used to signify "Inspected" ( Looks like a "T" with a slanted top.). Also seen on Cartridge clips, on the Bases of Larger calibre ammo ( 12,7mm and up), and a host of other Japanese ordnance equipment.

Early "No series" Koishikawa Carbine, probably saw China Service, given its "Ratty" condition.

Doc AV