That marking is a Liberty Cap. Figures quite prominently in the Argentinian coat-of-arms.
Haven't see it on anything else, but my collection is weak in that part of the world.
That is a bayonet adapter on the lug that accepts the ealier 1893/95 bayonet, the spanished used that setup on the M43/44 rifles. You can take it off and use standard M98 bayonets on it




























