Thanks for the link. They have the date partially wrong; left hand thread is correct as 1951 for J frames but it's 1961 for K and N.
The OP's gun has bluing on the grooves of the ratchets but the plate does not, and I believe I see brown soldering flux residue if it's not grease from when the extractor plate may have been soldered on.
The gun should be stamped "Not English Made" if sent out of the UK as surplus which it looks like it is but then X'd out.
It sure does look like they used a plate soldered onto the original star. Great pics.
What proofs & marks are on the revolver would indicate to me that it was Surplused ,,Sold from Service' mark >/<,,,but the conversion not done in England.
The 'Not English Make' stamp was a proof house required stamping.
It was placed on all non-English mgf firearms proofed at their facilitys (London and Birmingham).
Everything was proof fired, so foreign arms got that mark in addition to the required proof stamps.
In use from 1904 to 1925.
But sometimes you see it on arms proofed a little later than that.
The other proof marks are Birmingham View 'BV' (frame & bbl), Nitro Proof 'NP' (bbl), and third on the bbl I can't read but is probably the Birmingham Military Arms proof ''BM'.
All (each) of the proof marks are inside of a circle,, indicating the proof of a non-English made firearm.
On English made firearms, they stand alone,,no circle around the mark.
Then the application of the 'Not English Make' (corrected) marking to complete the process.
All those would have been applied when the revolver went to the English Military from the US.
If the revolver had been converted after surplus sale while still in England, the 22 conversion process would have required it to be presented for re-proof.
A new set of proof marks would be stamped on it indicating the caliber, again nitro proof, and depending on the time it was done, the cartridge length and pressure in tons/sq in ('54 law).
The laws and markings changed in 1925 and again in 1954, with updates to both.
Better it wasn't cluttered up with all those stamped markings.
It looks great just the way it is.
I like the 1917 and the 1851 repro conversions too. Custom projects always catch my attention.