The "Executive" typewriter predates the Selectric by a couple decades, and the "Executive" model was not an unusual business appliance in its day. I don't have price lists to refer to, but think that at the time the Selectric was introduced, the price of a Selectric and the "Executive" were similar.
Also, the "Executive" model had replaceable type-bars to facilitate making "special" characters. I don't know if IBM offered "superscript th", etc., but they may have. That sort of touch would "add class" to the correspondence of the day, and IBM may have offered parts to do just that. It's been long enough ago that it'll take somebody with an old archive of catalogs or manuals to find out.
I'm not saying the letter was done on an Executive, but to YOUR point that a Selectric couldn't have done it, I agreed. The Selectric lacks proportional font spacing. So, IF it was done on a typewriter, the typewriter was most likely an IBM Executive Model.
Having worked for the AF in the 70's, I can guarantee you that they were very skinflint when it came to new typewriters and every other kind of equipment in an office. No way would they have purchased anything extra for Executive typewriters when offices had to almost give blood to get them approved through the supply system.
But it can't produce New Times Roman Font in "True Type", which these memos have been identified by "experts" as utilizing. "True Type" has only been used in word processing.
LLS