First, if the 51cm sizing was for a modern slant top tube bike, it doesn't really translate. You are better off comparing the virtual top tube length.

In the 70s and 80s, bikes had level top tubes. You found your size by standing over them. Typical stand-over clearance was 1 to 1.5". You found this by standing over the top tube, and lifting up the frame and noting how far the wheels (both of them at the same time) came off the ground. Once that was sorted, you picked bike that had a reasonable top tube length for your physique.

Note that there was a trend towards smaller frames and larger clearance through the 80s, but people still rode bigger frames then now.