I have never seen the DT128 on any air cleaners other than new restorations. Probably came out around the same time the 400 Red Engine sticker people put on the lid. I have a bunch of pics on engine bays and it's a 50/50 split. Most are excellent restorations. Given the sticker is a "sticker" and it has color printing I don't believe it was originally put on 68 GM cars. Why is most if not all the was put on the engine area white painted and just a few items stickers? It probably was/is used on air cleaners but your guess is good as mine as to when it started. Just don't think GM had the color label capability.