I have associates within the FBI who tells me to be careful of using your own typewriter for doing nasty mail using other people’s names and addresses. Despite a lot of screw-ups in the FBI, their lab folks are sharp and can pretty well ID individual machines among mass-production runs of IBM typewriters.
The answer is either to use coin typewriter from a public library in another town, or to rent one from an office equipment store. Buy your own typing ball from another store and use it on the rental. Replace the rental ball when you take the machine back. “To be warned is to be wise,” and this comes from the FBI.
By the way, I assume you’ve watched enough detective shows on TV to know to use rubber gloves when handling paper and envelopes that are going to a mark. You just can’t be too careful about fingerprints these days.