This is my first time here.
I too have a 1905 "38 S&W Special Ctg". Marked U.S.N.C.P.C on the back strap.
And it has the same British "Proof Marks" as yours. There is a stamp on the butt (only half the stamp shows - bad stamp job) A circle with the word AID in it along with part of another word that I can't decipher.
So it must have been part of the "Lend Lease" shipment to Britain. I purchased it about ten years ago from a dealer who knew the owner who had inheirted it from his father who was a Policeman in Cuba. So this piece has been around. The finish on mine is not as nice as yours.