So from Jerry's observations, it would seem that we at least have all 67's, 68's and early 69's using the wire clamp, then changing to the stamped steel clamp, with the changeover possibly somewhere early 69.
In any event, the wire clamp on my 12c built 68 would seem right. [whew!]
Perhaps other early 69'ers could chime in with what's on their motor to see if we can narrow down the changeover date.