[simpleguitar]

He's talking about old mechanical style typewriters. Due to mechanical tolerances of the time, the letter shape and the alignment of the type bar would be slightly imperfect, and unique to that typewriter, acting as a fingerprint.

It's the reason why ransom notes from old movies are made of letters cut out from different magazines and newspapers. No handwriting, and no typewriter.

Did you know that US Secret service has samples of ink from various ball point pens? (https://www.12news.com/article/news/nation-world/look-inside...)