Actually, I believe that they could. The "font" on the old Selectrics was in the form of a type ball, which was replaceable. They could have substituted the ball at that point in the typing.
That's a ridiculous amount of work to do for a military records doc, though.
I used to have a Selectric. The font ball came in three flavors (that I know of): elite, courier, and lettergothic. No proportional, no kerning, no Times New Roman. You could move up half a line for superscript or down half a line for subscript but NO WAY did that change the size of the font.