Not Lend Lease but Maybe M1C Sniper???
The Distribution of the screw holes reminds me of the mointing screws used on the M1 C sniper equipment.
The proof marks ( barrel) etc are British Commercial proofs ( ie, the rifle passed "in trade" through the UK after it was surplussed from anywhere in the world) Nothing to do with "Lend Lease"
AFAIK, The US did not "Lend Lease" any M1 Rifles during WW II, but did "Military Aid" a lot of countries AFTER WW II. A small batch of M1s were used by Canadians ( at Dieppe) and reportedly a few were "Trialled" during WW II by British units ( ?Royal Marine Commandos?)
but it seems these did not come under formal "Lend Lease" arrangements.
Before writing off the screw holes as "Bubba" I would check the M1C screw hole arrangement to see if this is correct. Complete M1Cs are like hens teeth....Probably nearly all were dismantled when the M1 D (Barrel Mounted scope) camne into service.
Doc AV ( I have an M1D, but not an M1C....)
Down Under.