Dave:

Inasmuch as the part illustrated in your photos was not made to be attached to a ball stud (no spring loaded ball sockets), that particular avenue of search can be bypassed.
It is, obviously, of the nature of a turnbuckle; i.e., when the center piece is turned while each of the end pieces is attached to something, the end pieces are simultaneously moved either closer together, or farther apart. Such a device could be found in the throttle linkage between two carburetors, for the purpose of synchronizing them. The absence of the link would make the operation of the Zeniths very difficult indeed!

That said, it may very well not be from the car at all, but simply is something that became lodged on the manifolds years ago, and finally fell out when you disassembled the original carbs from the engine.

Re: Grinding noise

Was the vehicle rolling slightly when you placed the shift lever into "Park"?
How have you adjusted the control pressure linkage between the transmission and the throttle linkage?