there is a 303 sporter, I suspect british made, in auction now https://www.dorotheum.com/de/l/7444639/#

on the barrel marked with "Proven and Sighted for the Mark VII High Velocitiy Cartridge"

they have BSA in the description but this is maybe a guess. anyway, never heard of such a barrel marking before. will the mark VII cartridge give adate when this build happen?