OK this wasn't much fun but think I've got some information fro you to work with
One thing I would that even if the only side visible in the pictures was the side away from the label you could still make out the stitching from the opposite side to locate where the label was or had been in some of the pictures.
For the outer nonadjustable belt the label is sewn to the section that can be seen when the belt and rear seat lower cushion is installed as shown below
The inner adjustable belts typically had the label sewn to the belt in a location where the lower boot covers over the belt so its not clearly or fully visible and likely not visible at all when attached to the car and the lower rear seat cushion is in place. In a few examples the label was installed slightly forward where the label would then be just visible at the crack between the upper and lower seat cushion Examples below