Mine had the same carpet too('79).
I think it was about 73/74-ish when they finalised metrication so many speedos got them at about that time .. at least for the Rover vehicles I had.
I had plenty of P6B's, basically one of every year from 68 to about 74, except '70.
I remember the 73 had Miles, but the '74 had Km but with graduated Mile markings too.

