Just a thought but if it were actually stamped on thin wall with BSA markings it would have damaged the cylinder internally. That may be why it was engraved. I was told by someone that knows that BSA employed two highly skilled toolmakers working in a seperate department up until the 1980's whose job was was to make working prototypes for testing and evaluation. The design department had their ideas put into metal by these skilled chappies. The man with all the ideas was a certain Mr Roger Wacrow who had been involved in the design of the BESAL light machine gun patented back in 1943. He was in charge of the design of air rifles and pistols after WW2 the last being the 240 Magnum before retiring in the early 1980's. He was about to return part time after this but unfortunately was diagnosed with cancer. Judging by what I was told by someone that had dealings with him before he retired the thoughts that went into this pistol smack of his clever handiwork.