The 'Z' in that position merely indicates it is a service part number. In other words, a part to be sold by a Ford dealer and to be used to order a part.
The difference between 65-66 and 67 steering wheels is the thickness of the spokes and the type of grain/color on the simulated wood rim. Also, the bottom hub is different from 65-66 and 67... which is the main and most easily identifiable difference.
I also have one of the wheels in the box like you describe and chances are that it's a thick spoke wheel with the '67 grain and '67 hub. Technically, it's for a Fairlane, but can be used on a Mustang. It just doesn't have the thin spokes that a Mustang should have.