That each chamber is individualy proof marked is also an indication that it was civilian proof under the 1925 (theu 1954/55) Proof Law.
Proofing at either London or Birmingham was/is required before the x-surplus firearm could be sold on the commercial-civilian market.

When the Proof LAw was changed in 54/55, a single proof mark usually applied to the back face of the cylinder was adapted.
The 54/55 Law also changed the stamped bbl proof info to include the Caliber, the cartridge case length and the service pressure in English Ton(nes) /per sq in.

A few showing with the 'every chamber proof marked' style of the 1925 Law are around even though they have the later 1954/55 style proof markings otherwise on them.
Old habits I guess.

The Birmingham Proof Date code has been explained and a nice chart given above.
Use a magnifier to pick out the correct style and then the letter stamp & placement on yours and decode the Proof Year from the chart .

That's an awfully nice revolver and a great price,,congrats!!