what is this?

I'll stick my neck out and suggest it isn't just all 10" drum Mopar cars, but any (U.S.) car w/ basic Bendix brake floating shoe 10" drum design. Yours is the improved self-adjuster, which I put in my 60's cars. The "hardware kit" is a bit more specific because the "shoe pins" must be the correct length for your shoe width. Otherwise, I don't know that those vary between Mopar, Ford, or GM. Someone who worked in a brake shop should know and maybe chime in on Jap and Euro cars.