Thanks for the link Simon! There was a wealth of Boys info at that site.

Am interested in the sub-cal option. Anyone ever have the Parker Hale Brit 303 sub cal insert? A co-axis No2mk4 .22 would be a nice display and practice piece too!

It is a B serial prefix with an SA in square mark. Observing the stamps it may have had Indian ownership and been re-barrelled there in 1949 to .50BMG. That's what the rifle "says" anyway - the previous owners "story" was it may have been Finnishicon from the extension war with an unknown conversion to 0.5 BMG - he did fire live and blanks through it. No additional proof marks, no import marks so it seems to have got to the US a while ago.

Poor No32 BAR! I hope it was made better shortly after.