They are a standard size.
Yes, left, right, video, digital audio, subwoofer, or the data port on the side of the EXOCOMP from Star Trek - all the same - just different color insulators.
More likely simply using what they had - and FET's are still relatively rare and little used (compared with bipolar).I've seen & used lots of phono (as they are called in the Uk) connectors & never found one doesnt fit another.
Interesting techlib page, I see they used a darlington instead of a fet, maybe for electrostatic reasons.
Your label shows model BC-80.It's interesting how far back that connector goes.
My folk's had a Farnsworth console radio, built in the late 1930's, that had a "phono" RCA input.
I remember them buying a 78 record changer that plugged into the radio to listen to the old 78 shellacs.
Below is a picture of the chassis with the RCA jack just below the red label:
A modern RCA plug would plug in perfectly.