Revolvers that the British bought in a regular fashion through the British Purchasing Commission went to RSAF Enfield for unpacking and acceptance, and got the well-known stack of stampings on the left back frame and the crossed-pennant proof until fall 1941, when revolver shipments moved under Lend-lease.

But I do not know whether the batch of M1917s that were dispatched to Britain as a stopgap emergency help received that standard treatment. This one almost certainly belongs in that batch, and I can‘t detect any markings either.

PS: A quick forum search produced a few others with pictures, British post-war proofed but not acceptance-stamped, presumed from the same batch, so that confirms it.

20,100 guns in July 1940, straight from Army depots.