Well, I haven't been able to find anything good. Then best advice I can give is these 2 options:
- Find what is "factory stock" for the '66 C10, and work your way backward from there. That's not a very viable method I think because the best you're gonna get is a group of possible vehicles.
- Take it to a brake specialist and see if they can tell you. They probably won't be able to be more specific though, because many parts go one multiple models/years. They probably won't be able to tell you exactly which model year it can off of originally, but who knows!? Maybe they will!
The best guess I could give is that it's from a GM. It could be a replacement part made in a later year. The sticker looks like an 1980s kinda thing at the earliest. It has a barcode on it and I don't believe those even existed in the '60s. Could be newer too though.
I know this doesn't really give an exact answer, so if someone can come up with better information, I'm okay with that. But it's the best I've been able to put together.
Part of the problem is that (I think) there was a lot less serial number matching and all that back in those days, so it may be impossible to pull an accurate model year. I can say with ~95% certainty that if it was in another vehicle first, it was on a GM vehicle.