Parabola, chances are my re-bored .410 and yours are virtual twins made by the same person and sold on to different companies. Certainly looks the same in all respects. Mine was bored to take the 2 1/2" cartridge and shoots well. According to the Proof marks the conversion was carried out sometime 1939/40.

I too have a William Moore & Grey gun with a Thomas action which is a double 20 bore and number 1 of a pair. I am yet to find its companion. Lagopus.....