Standard Birmingham proof marks for a foreign made firearm (Proof marks inside bold stamped circles). The 'Not English Make' applied at the time of proofing.
These marks are of the 1925 Proof Law, valid through early '55 when the Proof Law of '54 took effect.
The 'Not English Make' was elliminated in the new proof law in '54,,the bold circle stamped proofs on foreign made firearms stayed.
This should have been done when the firearm arrived in England.
But the mark on the upper left of the sideplate is an RSAF Enfield Arsenal inspectors mark.
Quite possible the revolver was sent to the Brits at the start of WW2, inspected at Enfield Lock, and then simply put into service w/the Home Guard or where ever else they saw fit.
After the War, when these guns were surplused from the Gov't there and before they were sold commercially,,they still needed to be Gov't Proof tested. Possible then the full set of proofs were applied (before '55).
The one thing that goes against that post war proofing of the revolver is the fact that the individual chambers are proof marked. That's something that was generally not done after about the mid 1930's (each chamber was still fired w/a proof round as the law required, but the stamping of each chamber of the cylinder was not done).
But you'll find exceptions to that like everything else.
All speculation and a good guess of course..
The H.K.P. marking,,,I don't know. The guns from that part of the world seem to show all sorts of different styles of markings and most of them crude or hastily applied. Most of their handguns did have a lanyard attachment of some sort though from what I remember reading, and not all were of factory origin.
