Ok, I'm still waiting for a response from BP racing products in Italy, but I suddenly remembered that this was described as a Serpent Quattro brake-set by the ebay seller 2 years ago when I bought a Quattro and some parts.
So I opened the bag, and started puzzling....sure enough, it seems to be made for the Quattro!
Not sure which version (certainly not for the chain drive, it interferes) and I have not figured out yet how to attach the hub with the brakedisc to the rear axle. Maybe only suitable for the rigid rear-axle version? Or maybe just the version that has a gear driven rear-diff instead of chain or belt. The sprocket in the pictures was from my spare parts, but it is not for the rear-axle, it's for the 2-speed unit. But fits perfectly!!?!
It remains a mysterious part.Here are the pictures.
So, the conclusion:
Made and designed for the Serpent Quattro with the 'cascade' gear drive, nice idea, but low quality kit. I doubt if this will ever work properly. The backplate will need a lot of modification to fit.