If it was how did it find it's way back to the U.S.?

Theres no way to prove one way or the other that it was a lend lease but it makes the most sense. I know of Winchester 70's and Marlin levers with the same marks. The lend lease theory makes the most sense as the guns would have been sent back after the war ended.

It was law for the proof marks to be made to any firearm entering the country so that also makes sense.

The Birmingham marks are great because Birmingham kept records of the lend lease guns, so I could probably prove it for the price of a stamp.