People often wonder how far away date codes should be from the cars build date. They can be realistically probably from 6 months ahead to 1 day ahead. Some parts were built in batches. I pulled the following off the Buick site for you guys. It's mostly about the trains at Flint, it's a bit lengthy...but read the part about the Lear Jet at least.It is not my experience, but interesting! This would have been the same type of thing that happened at Olds plants, too.

It's funny what has happened to Flint. Today the Flint Assembly Plant gets switched, I think, two times a day. ALL they have there now is frames.
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In 1969 the Chevrolet Flint Assembly Plant was making 1970 model year Monte Carlos and Chevelles on one line and Pickup Trucks and Suburbans on the other.
------------------------------------------------------------------------------------------------------------------------------------------The Flint Assembly Plant back in the late 70's was making over 60 vehicles an hour on each of two lines.

There were FIVE tracks into that plant that was switched twice a day, 4 PM and then after midnight to be set for 6 AM.

Track 3 would hold about 14 or 15 random cars.
Track 4 about the same.

Chevrolet and Fisher Body were two very different Divisions of General Motors. The Chevrolet Flint Assembly Plant was north of the Fisher Body Division Plant. They shared a common wall. The bodies were made in the Fisher Body Plant and came through a hole in the wall into the Flint Assembly Plant. The bodies would be inspected and then "bought' by the Assembly Plant to be turned into finished automobiles. I don't know when they consolidated Fisher Body into just GM.

The shared common wall was between tracks 4 and 5. The tracks were side by side with a cement block wall between them.

These were in the part of the factory that used to be Fisher Body.

Track 5 would hold about 8 or 9 random cars.
Track 8A would hold 7 high cubes.
Track 8B 4 or 5 cars.

The Torrey Yard has 9 tracks. #1, the longest would hold 42 Bilevels to clear Van Slyke Road.

Tracks 2 through 9 were each a little shorter than the one before it.

There was usually about 150 loaded cars of auto parts in Torrey Yard for the Assembly Plant.

Fisher Yard was 7 tracks. There might be about only a few loads of auto parts for the Assembly Plant in this yard, but there would be empty T-1s for the Engine Plant.

Bristol Road South Yard would often have another 3 or 4 tracks of loads for the Assembly Plant.

So at any one time within the Flint Terminal there would often be between 300 and 400 loads of parts for the Assembly Plant.

Every night Train 388 would bring the block of high cube sheet metal cars from Indianapolis. There was ALWAYS cars en route that needed to be there by 06:00.

We called them "spotters."

The 07:00 Yardmaster would switch out and line up the spotters for the 16:00 switch.

The 15:00 Yardmaster would switch the plant and run the PD's and the SYDs. The PD's went to the North Yard to leave town. The SYDs went to the South Yard to be switched to locations within the Terminal. Example: Chevrolet Manufacturing, Chevrolet Engine Plant, Bad Orders, Belts - which were C&O Railroad cars.

There was a Messenger from the Flint Assembly that would hand deliver the lists of what was needed for the 06:00 switch. Or as was quite common, they might need a "shove-in" at lunch time, which was at 8 PM.

They had a count of every item in that plant. If they were running short of one part, they might need a car shoved in on track 1A, so they could take enough parts out of that one car to keep working the rest of the shift.

Now here is something to think about.

Everything that GM received by rail they also received by truck. Let me say this again. EVERYTHING THEY RECEIVED BY RAIL THEY ALSO RECEIVED BY TRUCK.

Except for loads of coal for the Powerhouse.

Very often they would need a "shove-in" because they were running low on one particular part, and they had some coming on a truck, but he might have broken down somewhere, or was just not going to be there in time.

This was all back before Just In Time freight.

They would have the inventory of all the railcars of loads of parts that were for them.

They might need ONE basket of a particular part shoved in....but they could have several cars with the same parts. They would look at the inventory of each railcar to see WHERE in each car was the particular basket of parts they needed and then give the Yardmaster an option of 2 or maybe more cars to choose from to shove in. They didn't want to unload any more than they needed to get what they needed and then the car would go back to the yard to be ordered in at a later date.

Believe this too. The Messenger would come to drop off the lists and he'd sit and talk for a few minutes. He was also their runner. He would make runs to Bishop Airport to get parts that were being flown in for them. The Messenger told my one day about the stuff they flew in for the Assembly Plant.

Items that were "difficult" repair items would be flown in. Usually a Lear Jet. One thing he said they often flew in was the connecting rod under the front windshield between the two windshield wipers. That was a difficult and costly repair item so they didn't want to run low on those parts. So if they knew they didn't have any in the Terminal, they would pay to fly them to them. Also one item that they could not have built fast enough for them was those 12 inch long brass ground straps. Those that grounded the engine to the frame. The Messenger told me that he very often went to the airport to get a package of those straps that he could wrap his fingers around. Flying in parts that he could hold in one hand!!!

A lot of items that were "easy" repairs they might ship the part directly to the dealer and have the dealer install the parts. But....the Federal Government said that ALL vehicles shipped from the point of assembly HAD to have the wheel covers with them. So they were flying in wheel covers every day.

The 23:00 Yardmaster would switch out and line up the spotters and get any that were in the South yard and bring them to Torrey Yard and then they would pull and set the Assembly Plant. And run the PD's and the SYDs.

The Flint Engine Plant had 2 loading tracks #9 and #10. I think they each held about 14 empty T-1s, 60 foot engine cars.

There was another track that received loads of castings. Crank shafts and engine blocks. And was used to load scrap. Mostly shavings from turning cranks and boring cylinders.

The Flint Engine Plant was putting out V-8 engines by the thousands. It was usually switched 3 times a day. And they would ship one or two loads of engines each switch to the Assembly Plant right next door.

The Chevrolet Metal Fabricating Plant was shipping over a hundred carloads of sheet metal parts and frames every 24 hours.
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Now you know why some dates are what they are!