The information over the block stampings is NOT just for the MGA and the 'B' series, it was used on the 'T' series XPAG,XPEG, The big MG pre-war saloons, WA,VA, ans SA, and so on. BEFORE 1953 and BMC
No-one really knows. Clausager says the G stands for mG; issue 1 of the BMIHT magazine says it stands for Garages. Since reading that MG and Wolseley joined Morris as a package I tend to think Clausa