I worked part of the way through college typing on an IBM Executive -- but I can't remember the model number and don't remember there being a "th" superscript key -- I rolled the carriage up when I needed to do it -- but that was 28 years ago and I could have just not realized the key was there, or maybe it wasn't there on all models. (Does this date me or what?)
What I do know is the first electronic fonts were based on common typefaces, like Times Roman and Courier. But the Executive type was/is actually a bit larger than the electronic software fonts. I think it's impossible to tell without seeing the original document what size the type actually is. I'm suspicious, though, of being able to get such an exact copy on MS Word --those things were a royal pain in the behind to use and I used to have to just freaking guess a lot of times when to stop -- you don't want to know how many little white out spots at the end of lines I used to have!!!
Anyway, like I said I think it is improbable but not impossible the document is real. I'd rather have an analysis of the ink and the paper -- the ink in particular I don't think you could fake.
I believe the little th was on some balls but not all. My old Brother (which is a disk system) is the same way...it is on some but not all. You want to look at an IBM KEYBOARD.