1984 Nishiki Century Rear Derailleur
 

Hey all I am new here and new to cycling in general (besides BMX riding), and I just purchased a 1984 Nishiki Century for $10 in need of a bit of love. I got her pretty much all cleaned up, polished the heavy a$$ steel wheels, threw some new treads and bar tape on. However, the rear derailleur no longer works, it moves to shift gears, but it doesn't keep the chain tensioned. The derailleur is a Shimano RD-Z503. What I am trying to find out is what kind of derailleur would work in this application, it's a 10 speed. I found this one locally on CL, and it seems like a good price, however I don't want to buy something that's not going to work. http://phoenix.craigslist.org/cph/bik/2426569961.html

Attached is a picture of what I found on the derailleur to potentially be the problem. There is a little nub that looks like it was snapped off which allowed the rest of it to pivot more than it should.

Help is greatly appreciated!
Matt