I wouldn't doubt that they're OEM parts. It's a good way to get quality stuff on the cheap.
When I was digging through the brakes used by the cars we have at work only the Bentley and Rolls-Royce brakes actually had the car manufacturer names cast into them.
Some of the Mercedes AMG brakes made by Brembo had the Mercedes star on them.
The vast majority of the calipers had no auto makers names on them, not counting stickers.

Since they made custom hubs to fit the car, chances are the rotors are off the shelf parts too. Again, this drastically lowers cost without degrading quality.

There's nothing at all wrong with using stuff like that. You can save a bunch of cash and add some serious upgrades to your 40+ year old car.