The only thing GM needed to see on the cowl tag (or the BBC hanging on the car is the body number, since that was the tie to the original dealer order that defined what options went on the car untilt GM assigned a VIN to it. After that, there was no info anyone on the GM side needed.
I've seen them as early as 11C in 66 all the way thru 06C in 67. A lot in the 03C thru the end of the year, but intermittent thru out the rest of the time. Not every original one has it, but its hard to tell if others that have had spraybomb engine compartment restorations have it. They definitely had paint applied here and there and figuring out why is what is called RESEARCH. There doesn't seem to be any obvious option associated with it, like every custom interior car got it, or every vinyl top car got on, or SS's and Z28's got it. They also started doing it for a reason, and quit doing it for some reason, as there are none in 68 or 69 with white paint on them. What changed in the process, again research, not anecdotal story telling. I don't know why they did it, but I do know it did Fisher no good, by the time it was painted.