Paul,
I know this is off the main thread topic, but here is my answer to your question.
The rears that I have seen date from mid Nov 68 to late Sept 69.
IMO the rear is too early and late for a 69 rear.
I believe the first ZL1 rears were mid-late Nov to early Dec 68 and the last COPO cars were early October 69.
Kurt S. should be able to give a more accurate opinion on this.
If the car is a clone, I wouldn't worry about it unless the seller is using the rear to boost the price of the car.
If so, ask to inspect/verify that the posi and r&p set are really correct COPO parts. If it has the wrong internals, the rear is at best 75% incorrect, assuming the housing is authentic (again just my opinion).
Hope this helps and wasn't too long winded,
Eric