AFAIK, the No. 4 rifles without the USP mark were made BEFORE the Lend Lease Act was fully implemented, when the British were still buying those rifles. The Lend Lease Act was passed in March 1941 but details of arms markings were not put in place until about September of that year. Production at Stevens-Savage began in July 1941, and some number were made before the USP marking was put on. The Lend Lease act remained in effect until the end of WWII, not just until the U.S. entered the war.