That's a Simplex Prestige rear mech, probably a 637 in those colours. A decent unit, and if not original then certainly period correct. IIRC the knuckles were made from an early attempt at plastic composite.
They had a reputation for cracking that was probably undeserved as many seem to have survived just fine. Clean and presentable ones can sell for north of 50 sheets.
Edit - they were made from Delrin, and the uneducated of the day simp,y got fixated on the word "plastic" and they were therefore automatically crap. Delrin is used for load and pressure (but not hot) bearing internals in guns, suspension bushes, all sorts of high load bearing stuff, so is probably over specced as a material for a mech.