I’m just sharing because it’s unusual, and definitely “vintage”.

For sale on Craigslist, a few more pictures here: https://cincinnati.craigslist.org/for/d/cincinnati-singer-240-3-chain-stitch/7377690264.html

The operator/adjustment manual is here: https://www.manualslib.com/manual/364492/Singer-240w.html#manual

It’s a simple chain stitch machine that only uses a single thread. ISMACS says it’s “for hemming operations”, and can apparently handle leather. A couple YouTube videos claim it’s for sewing hats, but I’m not convinced.

Video of one running: https://www.youtube.com/watch?v=fKIv4nW2taE

It’s interesting to me, because the only single-thread chain stitch machines I’ve ever seen are the vintage child’s Singers (I have one), and those hand-held bag-closing machines that aren’t useful for anything else. But maybe it’s a cool piece of history? It looks like it’s had some use. I might message the seller just to find out what it was used for.