I never say never but in the case of the wheels, with dimensional differences, I doubt they were mixed on one car. My grandfather worked and retired at Fisher Body, and my father had a very close friend that retired from the Norwood plant. Worked there from the 50's and into the mid 80's. Even toured the plant before they closed and learned a lot of interesting things with inside knowledge. Weird and unusual things did happen but a lot of this stuff is pretty cut and dry. After all this time passed, and original untouched cars getting more scarce, we may never know the answer to some questions.
My feeling, having a late December car with it's original wheels, I feel comfortable saying that at least the last couple weeks of December you likely had YH's. Before that time I don't think it's out of the question to have either AD's or YH's on a 69 Z. My opinion is that either would be acceptable. How early on the transition appeared is the question. With so few unrestored untouched cars from this time frame, especially non original owners, it may never be known.
