a couple of theories .... 1. i have worked in 3 different ford assembly plants over the past 11 years. the BASICS of the assembly process have not changed much in the past 40 years. i have had millions of parts pass through my hands from hundreds of different suppliers. the grease pencil is a very common tool used by many different suppliers to mark parts. could this be a unit count marked by a worker when he went on a break or shift change? when he would come back from his break or shift change he knew how many units were in his rack or container. if the grease was not removed before primer, the primer would not adhere to the decklid and leaving this grease mark. in the case of a metal decklid it would leave a rust mark because the primer did not adhere allowing moisture to get to the metal. i see these same grease marks with unit counts on them every day even today.
2. after final assembly and the vehicle is headed for the door, there is an operator who will mark the destination (also with grease pencil) for the guys out in the storage yard who will put the vehicles in their respective areas. some vehicles go by train some by convoy (truck), this mark could be the indicator as to where (which yard) it will be delivered. the problem with this theory is... why would it be under the decklid? when they drive the car out the door the decklid is down and the driver would not be able to see it. today they put the marks on the glass so everyone can see it, plus it will just wash off.
you would have to be VERY lucky to find the right assembly worker to explain any of the GT/CS questions. you would have to find a "utility" line worker or a "repairman" ,these are the guys that would know the many different aspects of this car. the average line worker back then only knew one job and he stayed on that job for many years. so if there are say 20 different unique parts to the GT/CS there are only 20 guys X 2 shifts = 40 assembly line workers (plus inspectors) who actually worked on the GT/CS. with only 6 months of production out of a 30+ year career of building 400 cars per day, these guys aren't going to remember a whole lot of detail about these cars. it is very boring and tedious work, sometimes mind numbing!
i like theory number 1 myself.
tom