The last READABLE Change Date listed in the 1968 AIM is June 3rd, 1968. As a matter of fact, there are several changes in the book with that date regarding the Exhaust Pipe and Muffler for the big block engines and the Z28.
Before that date, the latest READABLE date was April 29th, 1968. Again in several areas of the book.
Regarding the C60 option specifically, the latest READABLE date is April 26th, 1968. Once again listed on several pages. The seal in question is listed on page B3 with the following "Note 10":
Remove backing for installation
Going by these dates, this book was being updated all through the year and at least up to June 3rdth, 1968 with the C60 option itself being updated at least to April 26th, 1968. Therefore any car built after that "MAY" have had a change to the components or procedure not listed in the AIM.
Steve's car is a VERY late build. In fact, it's one of the last 68's to roll off the line. By that time, the 1969 AIM was out and it's "possible" the last few weeks of production for 1968 "may" have had 1969 changes applied to them.
JohnZ can you add anything to this?
By the way Steve, what's your VIN?