So what I'm getting is transmissions were built a year in advance of the assembly date stamp.
No, transmissions were built on a daily basis and shipped to the various assembly plants for immediate installation. A "typical" date will only be a few days to several weeks before the car assembly date. I think you're getting confused on the MODEL YEAR stamp.
For example... Let's say you have a 1969 Camaro assembled in October of 1968 (remember, the new model year started in August, so the 1969 Camaro actually went on sale in August of 196
8). The transmission would probably be built in September of 1968, but since it was destined for a 1969 car, the stamp would be P
9xxxx. The transmission year stamp was always the MODEL year, not the CALENDAR year it was actually built.
Does that help?
Ed