Hey guys, sorry for the delay! I haven't been at the computer in a while. By looking at the 'teeth' and comparing pictures of different tools I'm fairly convinced that it's a Shimano freewheel. Here's a picture of the assembly, for whatever good it may do;



I sprayed it with some penetrating oil and left it in the garage all night; I'm going to go out later and apply some torque to see if the cogs are spinning now. I'm not too optimistic though; I think maybe the suggestion of just replacing the freewheel may be a sound one. I don't know much about cassettes and freewheels, so I'm not sure what numbers like 13-28 and 14-28 indicate, but do you guys think this would be a sufficient replacement to toss on there?

http://www.amazon.com/Shimano-Tourne...6818843&sr=8-1

Or maybe this one? It's shinier looking

http://www.amazon.com/Shimano-510-00...6818843&sr=8-3