I know of a reasonably late (as far as we could determine without a fender tag April or May 1968) build '68 GTS convertible (number's matching) with the cast in timing marks. I helped pull the drive train and it appeared that may have been the first time. My 8/28/67 SPD GTS convertible that did NOT have the original engine when we bought it in 1983 had the early timing cover with the steel bolt on tab.