Wilbe,
I'm sorry to say I don't have a source you can go to at the moment, but because of England's inability to keep up with demand, the U.S. chipped in to build Mark 1 No.4 Enfields in .303 caliber. Do you have any more information stamped on the receiver that might tell you what U.S. Facility made it?
These are not the 1917 Enfields that were made in the U.S. in 30-06 caliber. esentually for our troops in WWI because of shortages in '03 Springfields.
Jim