I'm curious why they haven't done more forensic typographic analysis of the fonts themselves. There are a number of images of actual Selectric fonts, including [1], and that includes symbols and other variations. Close examination would either confirm the use of that equipment, or else discrepancies would be a valuable clue.
Thomas Phinney, Font Detective[2] is the first person I'd ask, as he's done this professionally, though I have done a bit of it myself.