If you look in the AIM (UPC4, Sheet B3 for 1968), you'll see that there was a cap (P/N 3908372) installed on the left rear shock on all first gen's except convertibles. I'm not sure why it exists, nor do I know why it wasn't installed on convertibles. Perhaps for some kind of protections to keep things from getting ripped, but that's just a guess. JohnZ can probably tell us the REAL reason!
Ed