Funny story...
As I mentioned, I suspect that there were 3 different rubber compositions. The hard blocks looked old and had kind of a brownish surface cast to them. They came stapled to the same white cards as the soft black blocks.
Maybe the rubber compound in the hard blocks aged faster and turned hard prematurely ???
I remember occasionally getting in recently manufactured bikes and boxed Mafac brake sets that the hard brake blocks.
I've had a 1980 Bertin C37bis with gold Mafac 2000 brakes since it was new. The brake blocks are the originals and they're still soft.
Earlier this year I acquired a very low mileage all original 1971 Gitane Tour de France. There's no way to prove that the brake block are original but the bike still has the original plastic bar tape, cables, housing and so on. The tires seemed to be the only things that were changed. The Mafac blocks are still soft on it too.
Chas. Colerich
Oakland, CA USA
> When I was a kid working at Turin Bicycle Co-op in Chicago in the late
> 1970's we had an endless supply of Mafac brake blocks. I mean boxes and
> boxes. The story was that sometime around 1970 when the guys running the
> store were often stoned, someone had ordered 500,000 pieces instead of 500
> he was supposed to have ordered.
>
> They were pretty hard by the time I was installing them as replacements in
> 1978. We did a lot of toe-ing in.
>
> Best regards,
> Marcus Helman
> Detroit, MI