"Ballester-Molina" Cal .45

[Johnny Peppers]

That is the original serial number. I believe it should be on the slide and the last three digits on the barrel.
The B prefix number is the British issue number.

[herd118]

20552 located on the backstrap is the manufacturer (Hispano-Argentina Fabricas de Automoviles SA) applied serial number. B.6537 is the British contract serial number.
Serial numbers range 12,000 - 23,000 were manufactured in 1942 - 1944.

[seebee1]

Many thanks for the explanation, much appreciated. I have never stripped the Pistol, not sure I would attempt it without a Manual, perhaps there will be other stamps-marks as mentioned. It is good to know about the date of Manufacture and that it was supplied to the British. Regards, Clive.