FWIW, my wife has been a secretary for a well-known phone company for 35 years, and has used almost every kind of typewriter of the major manufacturers that's ever been out on the market. She says she used a proportional-space machine back in the 70's, but the superscript feature simply wasn't possible unless you used an IBM selectric and went through the trouble of changing the ball in the middle of whatever you were typing. No one did that, in her experience.