Cartouche / Stamp ?

[MK111]

I assume that is United States Property on the reciever and barrel with the serial number.
If so it's a Lend-Lease to Britian during WW11. There was 42,000 rifles supplied. Your serial number in the 19,000 range was in the 3rd contract dated 3-27-42.
The British Proofs were installed when the rifle was supplied the civilian use.
Frank