[ladyjane]

I don't think it was even technically possible at that time, much less a decade earlier in the early 1970's

IBM had a typewriter with proportional spacing. If I remember correctly it was called an IBM Executive. i's and l's used 2 units of space; m's used 5 units. They were a pain to work with. They were available in the 60's and through the 70's.

[Willie Green]

They were available in the 60's and through the 70's.

I think the IBM Executive with proportional spacing was actually introduced in 1947.
The Selectric was introduced in 1961.
It was the one with the "golfball" typehead, not a daisywheel.
Also, it did NOT do proportional spacing.