Having trawled Skennerton it seems that if it were US it should have PROPERTY as well which means United States Lend Lease, WW2 but that was stamped on the left side of the action body.
Seems that the No:4 Mark1* was manufactured by both Long Branch and Stevens-Savage from 1942 but Stevens-Savage serials contained the letter C so should be easy to ID.
Anyway, there are far more knowledgeable people than me

here who will probably have an answer for you.