I realize that problems with Mafac brake squeal have been discussed in many previous of CR posts. In doing cursory searches of the archives I've not seen any mention of variances in Mafac brake block composition being an issue so please bear with me on this.
I assembled or otherwise worked on many hundreds of French bikes equipped with Mafac brakes during a 6 year span in the 70s. I also have a fairly large collection of bikes equipped with Mafac brakes.
We had some common sense fixes for Mafac brake squeal: make sure that everything was tight, clean the rims and brake pads, check the connections between the brake pad carriers to the round mounting studs and so on.
The usual fix was simply toeing in the brake pads. It remedied the problem most of the time. There were a few occasions during assembly of entry level bikes were we couldn't get the Mafacs to stop squealing so we replaced them with Weinmann brakes.
Just changing the brake block was an option too but it usually had little effect.
I found that there were three different rubber compositions used in Mafac brake blocks: soft black with low fiber content, hard black with low fiber and high fiber content blocks.
The soft black blocks stopped the best but had more of a tendency to squeal.
The hard blocks didn't stop very well so we usually changed them out. They may have been old stock that hardened up over the years but I saw brand new bikes with them. I have some Mafac brakes with 30 year old blocks that are still soft and stop very well.
The high fiber content brakes didn't stop very well either. They broke down very quickly, sometimes disintegrating.
Hope that this provides so new insight to the problem.
Chas. Colerich
Oakland, CA USA