A timeline history of the IBM Typewriter with old ads to show what the machines looked like. Click on the thumbnail to see an enlargement of that ad.
http://www.etypewriters.com/history.htm
Don't suppose you could just tell us if the variable spacing statement is true or not?
It seems there would be a finite number of typewriters used within the unit that authored the memos. Since typewriters have a unique signature, why not see if the memos correlate with any of the other documents done by this unit?
It shouldn't be too hard to establish given the volumes of paperwork generated by the military at the time.
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.