Welcome to the forum.
I've been wondering for a couple years if and when one of these would show-up.
You have one of the "Holy Grail" pieces of H&R collecting. Yours is only the second one I've ever heard of or seen. I have the other.
That gun was one of approx. 23,000 pieces ordered by the LONDON METROPOLITAN POLICE circa 1939 for war-time issue to the "Bobbie" or street patrol officers. Hence - this model is called (what else) THE BOBBY. The guns sent to London were serial numbered in their own range and derive from the final iteration of the AUTOMATIC EJECTING revolver (spanning the periond 1885 - 1941). There were also pieces made for commerial, domestic sales, which are serialed in a separate series.
The 'P' indicates proofing by the Metropolitan Police (MP) hence the stampings on the front of the grip strap. The MKII marking on the top strap, indicates the revolver has been proofed with Brit .32 revolver cartridges loaded with "Cordite" propellent.
Nobody knows (or at least they're not sharing info) how these came back to the US, or how many may have made the return trip - NOR - what happened to the 20K+ pieces that were shipped. Therefore - the rarity of finding one of the original MP marked pieces. Since these just do not show up for sale (as I said I bought one - mine at a gun show and the dealer thought it was just another JUNK H&R - so I got it for $135) valuation is nearly impossible to assess. In the BLUE BOOK, I estimate retail value around $600 - but to someone who knows what that gun is, and at a good auction venue - it could easily exceed that (IMHO).
Congratulations - would you care to share the details of how you came by that BOBBY?
Here's some pics of mine, along with a pair of commercial/domestic pieces.