The side marker change happened approximately 2nd-3rd week in February 1968 at the San Jose plant for Mustangs.
The first 200 GT/CS's were not all prototypes.
The introduction, debut, cars on the stage, are thought to be the hand built ones. Unconfirmed if some or all of those debut cars are designated GT/CS on their Marti Reports. Don't think that we ever came to a consensus?
As I recall, the first 200 units, were converted or partially converted, after they ran off of the assembly line.