Curiosity got the better of me...
With some of the talk going toward antisieze compound being used for anti squeal, I decided to analyze the paste from the brakes I replaced. The stuff is gray in color.
It's essentially made of copper flakes in an organic carrier. There's some aluminum and zinc in the mixture, but most of it is copper. It appears that it could be copper-based antisieze compound. Similar material is used for antifretting compounds used in bearing mount applications.
I don't know if the brakes are original as I bought the car with 52,000 miles on it. The thinnest pad has 3mm left from an original thickness of 11mm. (It contacts the sensor at 2.5mm!)
95 E320 Cabriolet, 131K