Briefly, the MG/FF was an older, drum-fed Oerlikon-type cannon whereas the MG151 was a more efficient, later belt-fed 20mm cannon developed by Mauser ( I think ). ‘MG’ was ‘mascinengewehr’ or machine-gun whereas ‘MK’ was ‘maschinenkanone’ or machine cannon ( I think )…..

Sorry about all the ‘I thinks’ – just off to work so don’t have time to look in any of my books 😮