So if I read the serial on the cylinder face correctly as V 745161, the gun originally was a late-war BSR from Nov./Dec. 1944.

It was definitely converted, refinished, and properly proofed for .22 at the Birmingham proofhouse. The stocks are not the usual British replacement style, but imitation magnas; I can’t make out the logo on the medallion.

I can detect no other markings indicating the converter’s identity.