One thought I have may be that sometimes cars were delivered to the dealerships without any engine size/HP decals on the air cleaner lids... as witnessed by many original cars, including the blue Yenko I recently posted pics of. I have a vintage mailout sent out by Chevrolet to their dealers, addressing the issue of dealers requesting replacement decals for cars during reconditioning, etc. and Chevrolet provides a brochure for an outside vendor who specializes in these "reproduction" factory-style decals. My thoughts are, perhaps dealerships used these aftermarket sources to add decals to cars that may have come in without any, in order to "dress-up" the engine and help the unknowledgeable salesmen and customers know what they were looking at when they raised the hoods... just a thought, as i still do not believe the decal in question was ever produced or installed by Chevrolet, but was indeed made during the same timeframe when these cars were new. I will attempt to post pics of the letter from Chevrolet, and the catalog... while the decal we are talking about is not shown, it proves there were outside sources for original style decals, even back then. A neat piece of memorabilia too! (click the link below)
Letter from Chevrolet