Earlier quoted context omitted.
The KGB used to require all typewriters to be registered, so they could identify the authors of anything they found objectionable. Being able to track who produced an image that's doing the rounds spreading "propaganda" seems like it adds a lot of value (from an authoritarian point of view, at least).
How could a specific typewriter be tracked from a typed document?
However, looking at a document, you may be able to tell the make and model of the typewriter, due to different typefaces and known mechanical differences across the make/model. That, coupled with a location or list of suspects, would allow you to examine specific typewriters. You may be able to determine a letter was written from a -given- typewriter, as machining differences in the keys, and differences in wear and tear over time can lead to minute differences in how the ink adheres. If you also have a letter written by a specific suspect, the variation in typing (how hard they hit the keys, common typing mistakes, etc) can lead to greater certainty of the author.