Ok took a look through my pictures and came up with some more examples where the head markings were visible. All of the ones I found used the wavy or toothed washers so that appears not to be a difference during the year and plant.
Removed my earlier post with the three examples so that I could post the pictures in order by their VIN in this one. In some of the pictures I also included the fender bolt also. Often they matched but not all the time. Expect that we will visit fender bolt markings in another thread in the future
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9R11107x S marking

9R111547 Head marking not visible

9R113512 Head marking not visible

9R115596 RBW counter sunk head

9R120249 Looks IMHO to be RBW counter sunk head

9R12028xx back to S marking

9R13593x Head markings not visible

9R14651x S marking

9R170501x May be the counter sunk RBW

Looks like so far there were two suppliers and we can start to see when each was available and used during different periods of production. Have one more section in my files to check for additional original examples
Just a picture of the wheel well side with the length and design of the bolt tip and nuts
