Under the driver's seat, under the carpet, under the tarpaper. Upper right corner reads: 1H57H2R605253.

Gues mine's a Monte Carlibu. It's for a 72 Monte that was yelloow with a tan vinyl top. Intersting. It was 153 ahead mine on the line, which I think is odd to somehow get back into a car.

It got dropped on the floor and by the end of the shift, the work area has to be clean, so the worker picked it up and threw it in your car.I worked at the Ford plant in St. Paul in '71/72 and once the trash cans were full all the crap went down the line in the cars...

nice.

That was along the guess I was going to make.  Somebody forgot to put it in the Monte and didn't want to get caught holding it, so they just stuffed it in another car as they walked by.

Seems like some of us assume that manufacturing cars back then was as controlled a process as today.  I'm sure a few things like this happen today, but not nearly as much with all the QC and mfg. process controls. 

I've been in mfg. for over 25 years, and every factory I've been involved with (both U.S. and overseas) cuts corners here and there or don't always follow the work instructions to the T.  That's how we end up with head scratchers like this.  Usually due to acts of human randomness! 

The line sheets (build sheets) were never intended to be left in the cars. I've always had the feeling that so many are found in seats because of the assembly process. The frames and foams were the same no matter the final cover color and it's possible that the frame and foam were set on a line with the sheet in view for the cover installer to see what they needed at a glance.

I understand the seat one a little. I found one in my original back seat that looked like a note pad sheet with the interior code on it, but what about the door panel, headliner, and gas tank ones? I agree that they probably weren't supposed to be coveted like they are (at least in more rare cars), but they are also put in places that are most likely to stay safe for years-well except the door panel. I suppose the gas tank one is taped to the top of the tank as it's installed and goes down the line since the body is not there yet, it can still be seen?