Selectric Typewriter Security Issues
Back in the 1970s IBM was required by at least one of its customers to make some options available for Selectric typewriters because they discovered the Selectric design was susceptible to electronic eavesdropping that could determine what the typewriter was printing. As it turns out, mid-’70s electronics were capable of detecting not only the unique electronic signature of individual typewriter motors but also measuring the load incurred by that motor during character selection from the main power feed outside the building. So, it’s possible that even using a mechanical electric typewriter for written communications may not be as secure as we think.
Typebar typewriters, both electric and manual, do produce a typeface “signature”; so actually does a Selectric typing element– the “ball”. However, it’s trivial to change elements. Were someone to use a Selectric for secure communications I’d suggest, depending on the level of security required, one typing element be reserved and used only for that purpose. The elements are metal-coated plastic and will burn, although there will be some metal remaining, assuming it is not melted.
Regarding Selectric-type ribbons, you have pointed out that the correctable film ribbon can be very easily read; so can the correction tape. Even the fabric ribbons can be read, although it’s much more work than the film ribbon. As an FYI, typewriters of all types will leave impressions on the platen (the rubber roller the paper goes around), with typebar machines, especially manuals, leaving the strongest impression. I do not know how long those impressions remain readable, but I know some impressions can be readable in limited form for a day.