Wish I could help you with a part Marc, but at least I can add to the knowledge base. My 68, which is original, does not have a date stamp on the rivet and has a large "U" on the base. Length is just about 35 1/2". It was an early production 68 model, so maybe they started stamping the rivets later in the year. Clearly, safety issues were accelerating in 68 and maybe that has something to do with date-identifying those parts. Who knows for sure? I do know that my convertible did not require shoulder belts, but later in the year they did. Also, I'm sure GM had a number of suppliers, which may account for some inconsistencies.