t reads GHD, not GDH. The initials are those of BGEN Guy H. Drewry, head of the Springfield Ordnance District. He didn't himself inspect guns, but his initials showed it passed government inspection. All lend-lease goods were purchased by the U.S., then "loaned" to the Allies, a legal fiction to evade the Neutrality Act when the L-L Act was passed in March 1941.
The British proof marks are commercial proofs, put on when the gun was sold out of British stores; most were bought by U.S. importers (e.g., Interarms) but some went to other countries or were bought by Britons, who at that time were still allowed to own functional handguns.
The .767" is the case length, so the gun is chambered for .38 S&W. The cartridge is still made, but is scarce. Many of those guns were reamed to convert them to .38 Special. Cases may bulge slightly, but it is a perfectly good conversion, since the barrels for both calibers appear to be the same, even though in theory, the .38 S&W should be slightly larger.
Technically, it is not an M&P, as that term was used by S&W only for the ones in .38 Special; the ones in .38 S&W for the Allies were called the ".38/200 Model".
Jim
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