How do you determine a car's final build date. My understanding is the cowl tag just has the body's final assembly date.

Generally speaking thats about the closest you can ever get to determining the build date. They built many cars and if the date is 12A (first week Dec) then is was sometime in the window.

The date of the trim tag was punched out the day the body was slated to start.

where did you come up with 12A...first week of dec?

Jan=1, Feb=2.....Dec=12
A=1st week of month, B=2nd week,C=3rd etc.

I just used 12A as an example of a built date on trim tag.