"As a Lend Lease item, it was expected or hoped that the revolver would eventually be returned to the United States"

There was probably no such expectation. For example, there were a great many .303 Enfield bolt action rifles made by Savage during the war, also with US Property stampings, for use by the British. I'm sure there was never any intent that such rifles would be returned to the US. In the same way, why would the US want a return of revolvers chambered in a non-US military cartridge? The Lend-Lease Act (1941) was primarily an act of subterfuge by President Roosevelt so that the US could maintain its neutrality status as it was legally not giving weapons to the British (and later other countries). However L-L continued all during the war, and the US property stampings continued even after the USA became an active combatant after December 7, 1941. I'm not exactly sure why this happened. Maybe someone else knows the reason for the continuation of L-L.

"But if the revolver were going to the Allies then why did a US Army inspector need to affix his stamp as well?"

Likely because armaments were being manufactured under US government procurement contracts and the armaments were technically the property of the USA. Virtually all items made under government contracts must be accepted by government insectors.