The black and silver number plates were 'old hat' by the early 70s. As said, the yellow rear, white front plates were available from 1968 and the majority of cars bought new would be fitted with the new design, particularly if it was a privately owned vehicle sporting the latest letter :)
I believe current UK legislation allows black and silver plates to be fitted to any 'historic vehicle' which is currently something registered before 1978. I actually have a real pet hate when I see black and silvers fitted to anything post 1970. It just looks wrong in my eyes, maybe because I remember the cars running around when they were just every day transport.
Original yellow and white 'Hills' aluminium plates with plastic rivetted-on digits look spot on on 1970-early 80s cars IMHO.