If you think the ANG had Executive typewriters back in the early 70's then I have some swamp land for sale. I have an executive typewriter in a box that I just dug out -- no "th" on the Executive typewriter. That doesn't even take into consideration that to get an Executive typewriter in those years you had to have special authorization and can guarantee you that NO ONE in the ANG was going to get that. Only General Officer/General Officer equivalent slots at MAJCOMs could get those typewriters -- the one in the Command Surgeon's office was ordered through the Medical Center because if the HQ put it in supply that would have caused an 18-month wait. The General in charge wanted OER's typed on an Executive typewriter as did the other Generals. Some guy in the ANG -- not a chance!
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.