Can anyone identify this markings on a .303 MkII* MG ?

It came form one of the 15 Lancasters which served in the Argentine Air Force until the mid 60´s. The meaning of the letters inside the circle has always intrigued me. I suppose that the crown, GR and crossed scepters means George Rex and War Office if I am not wrong. But I ignore the meaning of the BJ ( or is it a U or an O, very badly punched indeed ) and the back-to-back C´s. These markings might give me the approximate year of manufacture.

Cheers![ATTACH=CONFIG]219305[/ATTACH][ATTACH=CONFIG]219306[/ATTACH][ATTACH=CONFIG]219307[/ATTACH]