]My 1968 07c Z28 has the stamped steel.
Interesting...the only theory I can think of to make this fit with Jerry@chp's info is that the Z SSDI road tested was an even earlier build with the wire clamp.
There are some unknown variables of course. We don't know exactly when the SSDI road test took place, only that it was before Nov 68 and the build date of the tested 68 Z is also unknown.