I did not notice that this included another important text issue:
The questioned documents not only employ proportional type spacing, but employ "kerning" (more accurately, "pseudo kerning"), which causes certain letter combinations to overlap to reduce white space between letters in a word. The combinations "fo" and "fr" are found in the documents, with the top of the "f" overhanging the next letter. This capability did not then exist in any typewriter, including the special IBM typewriter that produced rudimentary proportional spacing.
Actually the TrueType employs a type of pseudo kerning using a lookup table for the space to the next letter based on the previous letter. Apparently true kerning which looks the same is when a piece of type has two letters on it. It was too technical to use when the proportional spacing was good enough to polish off the Killian letters.
For the true expert explanation see http://www.flounder.com/bush2.htm
However this site is sited in the FR Forgery Facts since that author is so eminently quotable.