There were many shipments made of arms and ammunition under Lend Lease, and shipments were handled in many ways. This gun could have gone straight from the factory to Britain, or it could even have been taken out of the hands of a US unit. In either case, it would have had all US ordnance and acceptance stamps, which this one has. These stamps would be missing only of the gun had been produced under a British contract, which it was not.
An example of equipment being taken directly from US troops is the 300 Sherman M-4 tanks from the First Armored Division shipped to the British in North Africa to help make up losses there. (This was the full TOE of 280, plus spares.) Because of this, the first Armored landed in North Africa under Operation Torch equipped with the inferior M-3 Lee tank. Roosevelt decreed that 1/2 of all US aircraft production was to go to the British, greatly slowing the growth of US airpower. Most aircraft arriving in Britain had to be modified with the removal of US oxygen systems and radios, among other items. I know, WOTS!
Anyway, absolutely beautiful gun! You'll probably want to locate the correct knurled pivot and safety, and a bright bolt, if the old girl doesn't have one. Congratuations!