10 and 12 point print was not necessarily the "size" of the typeface but the spacing....10 characters to an inch and 12 characters to an inch....closer spacing.
There were (as I wrote yesterday) certain elements (balls) on the Selectrics which had specific orders such as copyright, TM trademark and accents for foreign languages, etc. One could conceivably add a superscipt th on an element (ball) if one ordered that as an extra.
I cannot see it happening.... unless there was a mathematical font - element - ball which incorporated some mathematical and scientific shorthand...in which case one could substitute the element (ball) each time the superscript was to be used, remove it and return to the regular element for typing....
What I was saying, basically, is that there were different font sizes, but not THAT different. As you point out, it might be humanly possible that someone could order or have made a special selectric ball for scientific work or the like which could conceivably produce such a superscript of such a size, but the odds are probably one in a billion that a National Guard officer in Texas would have that equipment or would bother to use it in a private memo for the record which was unlikely ever to be seen by anyone else unless the s*** hit the fan sometime in the future.