reply to u/67comet at https://www.reddit.com/r/typewriters/comments/1e4hu0s/smith_corona_electra_120_ribbon_carrier_not_moving/

That piece is called a ribbon vibrator which moves the ribbon up and down. They generally operate on gravity and as a result they need to be clean and free from oil, gunk, hair, dust, etc. Usually they slide up and down freely. There's a colloquial saying in the typewriter space that "A typewriter isn't really broken unless it's clean and broken."

Occasionally ribbon vibrators can become bent which makes them inoperable and this can be remedied with some light forming (bending) with an appropriate screwdriver or needle nosed plier. You can search YouTube and you'll find a variety of videos for cleaning and forming these back into shape so that they slide cleanly.

As for your missing 1/! slug, it's unlikely that you'll find someone selling just the slug itself and then you'll need to solder it on perfectly and/or adjust it slightly with appropriate tools to get the right alignment. Far better is to check around with repair shops that might have the same or similar machines which they're parting out and then you could request one. Your best bet is to purchase the entire typebar and slug assembly from a donor machine which you can then swap out into your machine and skip the soldering. For this https://www.youtube.com/watch?v=pFMu6dUROGA can give you tips.

To find donor machines, try repair shops on this list: https://site.xavier.edu/polt/typewriters/tw-repair.html

How To Guide- Swapping Type Bars on a Manual Typewriter -Full tips and tricks by [[The HotRod Typewriter Co.]]