When I blow the picture up it definitely shows a BP marked in an oval. Apparently that mark is on every Remington-built A5 from the WW2 era as I've seen and read about it several times over the years. Maybe one of the resident Browning experts will appear and enlighten us as to what it signifies. What it is NOT; is a Remington date code.
Edit: I'm guessing that mark is some sort of proofing mark similar to the REP in an oval that Remington stamped on the opposite side of the barrel in the same position just ahead of the barrel extension on my Model 11 16 gauge from 1947 (Barrel Code SS).
Remington barrel codes for the period this gun seems to be from (1944-1946) were NN-1944, PP-1945, RR-1946 - at least according to Remington's reference. I qualify it because they could be a bit lax in coding barrels......my 16 wears the SS code on it's original IC barrel but I also have a MOD barrel which has no date code at all.......go figure.