The register in the photo does not look like cast iron. Not worth restoring.

Do you have both cast iron registers and some stamped metal ones?

As said, those are metal not installed with the initial construction date. If you don't mind the bumps and stuff, scrape off anything peeling and just paint over it. In the past, I've been able remove a couple bumps (paint blobs or paint over dustballs and hair) by scraping them off with a blade, but it looks like you have too many and on the fins too. If you wanted to have pristine grilles, you would have to remove all the paint, which would be tedious by using chemicals or blasting, so it wouldn't be worth it.

Thank you all. You’ve saved me a ton of time!!!

Maybe soak them in stripper to get old paint off and softened. Wear gloves and scrub off paint. Maybe slightly tedious but not that bad. Use a wire brush to remove some of the rusty spots. Then repaint with spray paint, use a primer. They are probably better made than the new ones they sell now. Also may be hard to find the correct size, possibly.

I wouldnt put the effort into doing anything with them. I am sure you have a lot more to do with your time, and the money for all the stripper, and all the work, than to mess with them. Buy the ones from the Home Depot, or even a regular hardware store. They are just old metal, like the new ones you can buy. You can even buy them now, with a hardwood design on them, that will blend in with your floors.