it is more likely an individual's interpretation/use of that "Date Code Conventional Wisdom" that is not correct

starting with an 18-spline transmission isn't a great example:
1970 18-spline vehicle production ~12,000 units (~1,000/month) VS. 1971 18-spline vehicle production ~1,600 units (~130/month) - total at 4 assembly plants = 18-spline was slow moving inventory during the 1971 model year

where was Chrysler Corp.'s New Process plant that assembled these transmissions?
can we "ASSUME" the "PP833 DATE UNIT" numbers were stamped when the transmission was completed as an assembled transmission - transmission plant
........... & the painted last 3-digits of the assembly (transmission & shifter) part number was added after the shifter & linkage (or at a minimum the shifter mount?) - transmission plant
........... & the partial VIN number was stamped when the specific transmission was selected for a particular car on the car assembly line - car assembly plant

and there was NO "just-in-time" or "first-in/first-out" inventory practices in play 50 years ago at the manufacturer or assembly plant

aren't the transmission side cover/detents "NEW" for '71? what side covers are on these two transmissions?
the LA transmission has "755" painted on it - this was for '71 18-spline B&E-body with bucket seats (with or without console/non-CAR), is the Lynch Rd. transmission from a bucket seat non-CAR car?