reply to u/SlumberCrow at https://reddit.com/r/typewriters/comments/1orwxqq/type_writer_leaving_small_divets_in_paper_when/

A new platen will certainly help, but it's also a question of having a proper ring and cylinder adjustment across the length of your platen and segment. Often letters that punch through tend to be the . , and o which are at the extreme end of the segment. Some machines have adjustment screws at either end of the carriage and the adjustment should be checked at not only the center of the platen but both ends. If you don't have an experienced mechanic who knows how to do all of this properly you can easily get issues which will most often show up at the far ends of the the segment/platen.

Beyond a proper adjustment, it's also the case that the surface area of the . and , are smaller than other characters and so they tend to get more force even when actuated by the weaker fingers on the right hand when touch typing. Some older manuals and training films will suggest putting less pressure on these keys when typing. This is likely even more important for those who hunt-and-peck and are likely using the full force of their index fingers.

Unless your ribbon is obviously dry or marginal, replacing your ribbon isn't likely to help much. Slugs are made out of hardened steel and you'd have to do something incredibly drastic to damage the slugs, so don't sweat that too much. Backing sheet will help as a stop-gap particularly on machines with older/hardened platens, but there's only so much help that will do without a good platen and a properly adjusted machine.