Originally Posted by
Grand Bois
I think you're right. That bike was built after they switched to a standard Campy-style hanger. I'd replace the Simplex with a Suntour whether the Simplex is broken or not. The SX610 is good. Suntours are great.
This.
For the OP
The Simplex SX610 is a nice derailleur but when this was built this design was still a few decades behind Suntour in the performance department.