Current modern (as of just a few years ago) C&U regs require one offside mirror, plus one other mirror - this can be either internal or on the nearside.
No idea what the rules were in the 70s, but I can probably find them...

Graham (G4FUJ)

Sold L44FOR 4/4 Giallo Fly
'09 Gen2 MINI Cooper ragtop
'90 LR 90 SW