Hi Skybolt.

Congrats on your new toy. Not my area of expertise however your proof marks are explained in http://proofmarks.tripod.com/englishproofs.html . Go down to the section "British Proof Marks 1868 to date" In particular the column for London proofs. My limited interpretation is that it is only proofed for black powder not nitro.

Unfortunately it dosen't give any illumination on the "40 S" markings.