I've had four Zs to compare against (early 71, 72, 76, 78) and from what I can tell that's not really true. The series one cars do, in fact, have thinner metal (about 0.9mm vs 1mm for later models), but otherwise the metal thickness as far as I can tell is generally the same. What you will find in the later models is additional reinforcements used, which means more metal.