I've done a little research into the Registry which is here online and the Archival Registry which I am currently the "Keeper of the Registry" records. Of course, these records do not have all the information needed for a complete analysis. Meaning that; the owner did not include the information from the firewall trim tag.
Typically, on a hardtop or post car, one would first read the body color (letter code) followed by another letter code to indicate the top color, whether it is painted or a vinyl top. For convertibles, the first letter is the body color followed by a number code to indicate one of the four colors of convertible tops available. That being said, the 1965 Buick GSs that were built in Fremont, California do not follow this pattern. This is not unusual as the Fremont plant's firewall trim tag is entirely different from those '65 GSs that were built in Flint, Kansas City or Baltimore. Generally speaking, the paint code on the Fremont cars either had two letters or three with the last letter always an "A". Examples TT, CC, KK, CC A, RR A, ZZ A. Here is a short list of the Fremont built cars and their codes as recorded in the '65GS Registry. BTW, a Fremont built car will have a "Z" in the VIN.
YY A = Coupe, Bamboo Cream, Black vinyl top
CC A = Coupe, Artic White body and painted top
CC A = Coupe, Artic White body and painted top
AA A = Coupe, Regal Black body and painted top
KK = Coupe, Turquoise Mist body and painted top
VV A = Coupe, Shell Beige body and painted top
CC A = Coupe, Artic White body and painted top
TT = Coupe, Champagne Mist body with Black painted top
TT = Coupe, Champagne Mist body and painted top
CC A = Coupe, Artic White body and painted top
ZZ A = Convertible, Silver Cloud body with White convertible top
RR A = Convertible, Flame Red body with Black convertible top
CC = Convertible, Artic White body with Black convertible top
I wish I had more complete data from our Fremont, CA built cars, but this is enough information to see that they either had two or three letter codes. They seem to ignore the convertible top color number code altogether. The first paint code just appears to be redundant. The last A code could be the standard silver wheel insert color (silver/ gray) as Walt pointed out, similar to the Olds 442 in '65. BTW, the Oldsmobile 442 was also built on the same assembly line as our GSs.
That's all I have for now.
John Egel