Can't remember if I posted this when I got it, but I just uncovered it in my car tools again and can't remember what it's for. About 5 1/2" long, hex opening looks to be about 13/32". The only writing on it is RAP  612. Any ideas?

Yes. For adjusting valves.

Yes.  Interesting variant of the tool.  I've got one of the basic versions, which takes interchangeable standard sockets.  Mine's got a flat screwdriver tip, though.

They're said to be dynamite for Chevy small blocks, which are adjusted while the motor's running.

I think it's for rear bands on auto trans.
Slot to wide for valves
You loosen the lock nut , tighten down the adjuster and back off a certain amount of turns.

That might be more likely. The adjuster for valves would be "male" rather than "female".

Great!

Can anyone use it? It's just gathering dust in my car box.