The IBM Selectric which we used extensively in the early 1980's was a daisy whell device which did not use proportional spacing to the best of my knowledge. I don't think it was even technically possible at that time, much less a decade earlier in the early 1970's. Also, I was a training sergeant and buncked with the company clerk. It is true that rank abbreviations were done exactly one way and 1LT is correct for first louie.
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.