Originally Posted by chuck731

Total repair bill: $215 for the replacement Record short cage, and $50 for a new chain.

Apparently the back plate of the derailleur cage is not sold separately. To get that you need to also the rest of the cage mechanism, including the CF front plate, the tension adjuster, the pulley wheels, the ti bolt holding it to the derailleur body, and the spring.

I can almost get a entirely new derailleur.