I think it depends on your intent. What if your original tag is badly rusted but legible? I see no problem getting a copy of that tag made and putting it on your resto.

And part of it is terminology. Why is it every other piece on the car could be fake (excuse me, I mean reproduction) like fenders, hood, emblems, etc., but a repro tag is not a repro, but fake? When you sell your car, do you tell the buyer this and that are not original parts but fake ones?

Again, intent. I would have no problem with someone wanting to create a tag to "complete" his car. I would have a problem with anyone trying to pass off that car or anything else, be it a gem or a painting or a Rolex, as something it is not. The more rare the item, the more people open themselves up to getting ripped off.

If you are willing to pay $100k for an original 6-pack RR (or a gem, painting, etc.)you had better be an expert in that particular field and well-trained in ascertaining whether or not that particular item is an original. Or employ someone who can. If you just take some Joe's word (dude, this is a real Rolex! Seriously!), you deserved to get ripped off.

Documention on these cars was never meant to be as hard to counterfeit as a $100 bill. Maybe instead of wondering if a tag is original, you should just, like a retailer checking your $100 bill, assume all tags are fake and pay that price for the car.