I very much doubt that the "DP" mark and the hole are actually related.

Presumably the rifle was DP'd in service. Looks like the rifle as then had another shooting career - if that is indeed a civilian BSA ball-burnished barrel fitted, as it appears. Someone has then decided to make it into a display rifle with a rather stupid "invisible" de-activation.

This incident again makes me wonder why the US and some other countries have no system of proof or other stringent check. At least in UKicon and Europe you can be reassured that your salvaged DP or home-made conversion is safe enough by submitting it to an independent over-proof test.....