According to the
IBM archives website, the IBM Executive Series typewriter with
proportional spacing was introduced in 1941.
IBM announces the Electromatic Model 04 electric typewriter, featuring the revolutionary concept of proportional spacing. By assigning varied rather than uniform spacing to different sized characters, the Type 4 recreated the appearance of a printed page, an effect that was further enhanced by a typewriter ribbon innovation that produced clearer, sharper words on the page. The proportional spacing feature became a staple of the IBM Executive series typewriters.
I would still question the authenticity of the actual font face used and the "th" superscript, but it doesn't seem impossible that a 1972 document could have been produced with proportional spacing.
I checked this out with my "expert" (Google), and he came back with this link
http://www.ibmcomposer.org/SelComposer/description.htm(with pictures). In 1966, IBM began producing a version of the Selectric, the "Composer", that could do proportional spacing, but it appears to have been a high-end, professional model that wouldn't have been very commonly used.
Apparently, you had to type a line, record the number and the line's length separately by hand,
then re-type the line with the number and line length inputted into the typewriter. Basically, it looks like an extremely top of the line typewriter that an average person would not use, especially for a note to one's own file. I doubt most secretaries knew how to use that thing, let alone an Air Force officer. Check out the link for the pictures, that thing looks un-Godly complicated.
Thus, I still think the proportional spacing font indicates that these memos are forgeries. Perhaps some experts could weigh in?