How about another curve ball thrown in...
This may be reaching but I see it entirely possible (this happened alot with fan caution decals in the Ford camp when cars came in for service) Since alot of these cars didn't sell right away, alot were sent back or distributed around the country at various dealerships....Isn't it entirely possible that those dealers, or even Gibbs himself saw fit to run over to the parts department and stick "427 decals on themselves when the cars were only days or weeks old??
I mean, without the decal, and no identification on the outside of the car, I'm sure there were alot of people back then that didn't know what they were looking at, and could have easily mistaken the cars for 396's that cost $7,000. Since they were a hard sale I could see dealers doing things to push these cars out the door. The iron block COPO's had air cleaner decals.....so could it be possible that dealers thought,,,,hey why not??
Just a thought, and speculation on my part, but a possibility. Either way I certainly wouldn't kick it out of the garage just because of a $6 decal. Just buy another lid and switch it back and forth. If you can afford the car, you can afford another air cleaner lid
What I do like about this hobby is that there are still original unrestored cars out there that show things weren't always cut in stone, and some people aren't afraid to restore a car that doesn't follow what is widely accepted as correct. William has posted enough information to suggest that sticker or no sticker is entirely possible. What pains me are restorations that are done on original cars, and things are changed just because that's what is accepted in the main stream or due to peer pressure with judges and the like. I have an unrestored car here that has a few oddities about it, but are viewed as entirely possible by other "supposed" experts that have seen the car. So I don't necessarily see this decal issue as incorrect.