I am intentionally being specific about the Army's first version as being unmarked. These were the finalized form of the General Purpose knife, as approved for use by all branches of service. Kingston also made a prototype version (screwdriver/cap lifter and can opener blades were reversed, which did not work out) for the United States Marine Corps that was marked "US Marine Corps," which was produced a little earlier, starting in late 1943. The Marines had a bunch of these, so did not switch over to the standardized version right away
The handles start getting stamped US around 1948-49. Blades start to get year stamped at about the same time. The blades, liners and springs went all stainless a bit later.