I'm sure there's an answer for what it was
supposed to be, and equally sure the reality is you can find it applied above, below and directly on top of the body line on any number of original cars.
On my '74 Duster all 3 of those positions are accounted for. On the passenger quarter the moulding started at the bumper end below the body line, with the top edge of the moulding at the body line. By the time it reached the passenger door, it was above the body line, with the lower edge at the body line. On the drivers side it started at the rear of the quarter above the body line, and transitioned to directly on top of it.
Drivers quarter, clearly above the line at the tail of the car
Passenger quarter, above the line at the door and transitioning to below by the middle of the wheel opening
Close up of the witness mark, above to below the body line just behind the door
Further back on the passenger quarter, witness make entirely below the line, top edge even with it.
So yeah, clearly the speed of the application was more important than its accuracy. I'm also sure you will find purists to swear up and down they were all the same and the way they think it should have been. These were probably applied by the least qualified person on the entire assembly line, because they required zero actual skill to apply. So the reality is there's what was in the book, and what happened was that and then everything else possible. Think, if you gave the job to a 19 year old and told him he had a minute to do the whole car.