More info:
Build sheets:
66 A code GT, 38A, HDB (heavy duty battery)
66 A code GT, 38A, HDB
66 K code GT, 42A, HDB (code 2)
66 K code, non-GT, 42A, HDB
66 A code GT, illegible, HDB (code 2)
Window stickers:
64.5 K code, no HDB
65 K code, non-GT, (5) without HDB, (1) with HDB
66 K code, GT (2) with HDB
66 K code, non-GT, no HDB
This doesn't really help with much other than maybe starting in 66, the 42A became a requirement for K codes.