You ask why the superscript raised the poster's eyebrows? Because there were NO tiny little t-h on the typewriter, even if it WAS a proportional spaced font--something I never saw in the early 70's and I was in the typing business.
Another red flag is the perfectly centered name and address at the top of the memo. It was hell on wheels to center anything manually on a proportional typewriter, whenever they became used. This is centered perfectly, very very strange, IMO.
In the 1970s, every typewriter I used was one of those big black manuals, and I was in aerospace. Government lagged behind all technology in anything that had to do with paperwork, didn't think it necessary to upgrade perfectly good equipment for some newfangled gizmos. We used carbon paper back then!!!! If there were copy machines, I sure didn't have them in my offices.
While it is possible that this is the real deal, the little "th" the poster mentioned is a huge red flag, there was NOTHING on ANY typewriter or word processing-type typewriter that had ANYTHING even remotely like that. The use of that is real recent. When we wanted superscript or subscript, we just rolled the platen up or down, typed what we wanted, then moved the platen back. Even word processing documents, until recently (thanks to Microsoft and I think the little "th" and "rd" looks stupid anyway) didn't have the capacity to do that without a lot of work changing the size of the font, etc.