When Chief Ford (Clifton James) comes to Mannix' motel room, Mannix receives a telephone call. After he hangs up, Mannix scribbles something on a notepad with what is clearly a pencil (and which makes a "scratching" sound as he writes). When Mannix leaves the room, Chief Ford picks up the note to read it, but the note, which is shown in a close-up, is written in ballpoint ink.
After Celia Bell (Barbara Rush) gives Mannix what is supposed to be the missing last chapter of the book Mannix is searching for, he compares a page from that chapter with a page from an earlier chapter in her motel room. These pages are typed with a serifed font (common on manual typewriters of the era) similar to Courier or Prestige Elite. When Mannix takes them to newspaperman Archer (Kevin Hagen) to examine them with an overhead projector, however, the pages - although the content is similar - are now typed in a non-serifed font similar to Helvetica or Arial. (Furthermore, although non-serifed fonts would have been available on early versions of the IBM Selectric during that era, they would not have been found on a manual typewriter without interchangeable typeface elements.)