The letter 'blanks' which hit the ribbon in a typewriter are imperfect, as all physical objects, so having a sample and a registered owner can be used tell that

The letter 'blanks' which hit the ribbon in a typewriter are imperfect, as all physical objects, so having a sample and a registered owner can be used tell that this document was written on this particular typewriter because e.g. all their letters 'F' have a tiny scratch in just the right place. It's similar to e.g. identifying specific guns by the markings left by imperfections of the barrel, you'd also take sample shots which can then be matched to the specific gun.