Here are a few more pics for everyone’s viewing pleasure. Please chime in if you have any additional insight or if you want to correct me for being dead wrong! Based on its features and stampings, I would say this is likely a Lend-Lease gun from the British order of 400,000 units in 1942. US marked, no Brit stamps, L sight, vertical grip, cutts compensator, etc. The interesting thing to me is that it looks like it was never “adapted” for British use. No forward sling swivel was ever added to the vertical grip, and the buttstock swivel is still located in the original position at the bottom of the stock. In my limited research, I understand that the brits almost categorically moved the swivel to the top of the buttstock, while mounting a forward swivel to the (usually right) side of the vertical grip.
Of course the possibility exists that the wood furniture was added later, but it sure looks like original to the gun. My inclination would be to think that this was potentially unissued, but at the same time it seems unlikely that the Brits would pay such an exorbitant amount for each Thompson and not get use out of every one of them. Please educate me!