Thus, for a consistent looking document in those days, manual typewriters required that you hit each key with equal force, and steady speed. But that was difficult if you had varying strength in your fingers, and varying speed skills. Novices could often be identified by lighter "a" or "q", versus darker "j" or "f" characters, for example, because of the different strength of the pinky finger vs others, etc.
Also, varying skill and your training could result in having "favorite" keys, versus keys you weren't good or as speedy at.
Also, the alignment of the character hammers would degrade after a while causing spacing errors and characters that were darker on one edge than the other. Some would get so bad as to open up a "O" making it almost a "C". If you see what I mean.
Also, negligent typists, who didn't clean or brush their keys regularly, would suffer ink buildup on the hammers causing a variety of distinctive characteristics.
In summary, a document produced by a manual typewriter is similar to a human fingerprint. Serious investigators can easily match the document to the typewriter and to the typist in an office. A lost art now, methinks.
Excellent summary!!! Saved me all that “typing”!! Thank you.