Check out these 2 manual trans date codes. According to Chrysler's 10,000 day calendar, both these trans were built on February 4th, 1970 and should have been installed in a 1970 Model Year B body. Yet both were installed in a 1971 Charger and GTX. The Charger SPD was February 1971 and the GTX SPD was March of 1971, both about a year after the transmissions were assembled. The mopar conventional wisdom is that these transmissions should have been installed in April or May of a 1970 Model Year Car. Notice too that although built on the same day, they went to two different manufacturing plants (Los Angeles and Lynch Road)..

After seeing this post...I wont be able to sleep for a week......

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?

There are numerous items that have been brought up on numerous items that defy conventional wisdom. If anything, we should know that you can never say never when it comes to classic mopars. While conventional wisdom may hold for a vast majority of cars and parts, there will also be those outliers and exceptions that can be found.

I wouldn't loose any sleep over it.

LA did not produce any E-bodies for '1971.

(LA E body production apparently halted some time in May of 1970)

I don't have a picture of the cover side unfortunately, but both cars were 1971 B bodies with bucket seats.

6bblgt - In you post, you mention "with bucket seats (with or without console/non-CAR)" What is "non-CAR"
I'm impressed with your knowledge of the finer details of transmission numbers and assembly plants. I assume the same goes for engines. Engine assembled, stamped with date code of assembly on top pad behind water pump, then put in que to be shipped to the car assembly plant were the VIN is applied at the same time the VIN on the trans is applied, correct?

CAR = center arm rest

The 833 from my U1G 71 GTX SPD 126 is 3323 Sep.2 1970,71 style side cover.

Just like Hemi blocks cast in early 69 and installed in 70 and 71 cars...

Date coding isn't an exact science. ANY of the date code guidelines are just that, guidelines that reflect what typically happens. None of them are hard and fast rules.

Just an observation: in the late 70s I worked at the Kokomo Transmission Plant. There was a large area where they staged the loaded racks of finished transmissions waiting to be shipped out to the assembly plants. Sometimes this area was pretty empty, so the transmissions would be shipped out very soon after completion. Other times this area was completely full with racks of transmissions back against the wall, completely blocked in. In this case the recently finished transmission still went out quickly, but sooner or later the transmissions that were back against the wall finally went out to assembly plants. The production date on them would be much earlier than the assembly date on the car. First in first out was probably a goal, but many times the cyclic nature of the industry got in the way.

I had a '71 GTX built in St Louis in Feb of '71 and the engine was assembled in June of '70. But all of the accessories were dated immediately before the car's production date. Most within a week.

^^^ Exactly. People forget that assembly plants were just that, assembling the cars from parts & sub-assemblies provided.
The manufacturing of those parts & sub-assemblies was elsewhere and for cost-containing & logistics reasons, would be done in batches.
Side note: "Just-in-time" wasn't often practiced in the US at the time (1st introduced in Australia by BMC, though largely credited to the Japanese).
So, many if not most '68 Hemi cars had blocks cast in '66, as one example.
I'm sure that process varied among sub-assemblies (dashes for instance) owing to storage space, vendor procedures, and the need to keep the assembly lines flowing.
Places where raw ore came in one door of a facility and a finished car came out the other were & remain rare (Ford's Rouge megalith).

This continued well into 2000 and later, the Grissom Air Force Base near the plant was used to store production transmissions.

Most people think there is supposed to be some tie in to the production date of a vehicle versus the built date of components.

The build date on an assembly or component is for others purposes. While it helps to keep component dates close to vehicle build date it means nothing in the end.

Interesting Dave. Assembled 2 months before the beginning of the next model year. Do you recall if the engine pad was stamped F440 HP or G440 HP?

It was 'G'. I am as sure as one can reasonably be that the engine was the original. The car was a solid car that had sat for years when I got it. All the accessories were original, and not even the valve covers appeared to have been off it. You could even still read the bore marking letters on the sides of the block. Greasy and nasty in all the right places. The car was just a garden variety 440 auto, nothing special and at the time they weren't bringing big money. So there wasn't much incentive to doctor the numbers.

Very Interesting. I'm not an expert but I believe the G440 HP or F440 HP (or whatever year) is stamped at the same time the assembly date is stamped on the ID pad along with other engine identifiers like undersized crank or oversized lifter bores. If the stamp was dated June of 1970, and it was stamped "G" 440 months before you could even order a 1971 Plymouth, could it have been a preproduction or pilot car?

Wouldn't think it would be a pilot or early car as a February build.

My guess would be that at that late date in the model year they just waited to stamp the model year like they did the VIN. That way it could go into a car or across a parts counter. Or maybe it was a parts engine to begin with that got put into a production car for some reason. It's Mopar and it's mass production, who really knows?

I'll throw my guess in here. If it was near the end of the 1970 model year and there were sufficient assembled engines in stock to fill the remaining 1970 orders (or close to it), rather than shut the engine plant down for two months they simply started building engines for the 1971 model year even though it hadn't started. As it appears the engine sat in stock for nearly a year before being used tells me that demand for the 440 was falling OR it was a victim of FILO inventory. Which brings up another topic. There have been numerous posts on how to preserve an assembled engine for long term storage. I wonder how the assembly plants handled this?