It's a Mavic 700c Brake Post Adapter @Gaston. Been looking for one for a couple of years with no luck as they are no longer in production. Xtracycle produced some as well. There are some BMX brake adapters available that might work and other mods you can do. ACS made a cantilever brake adapter for 700c and 650b in the 80's, using MAFAC tooling after they bought them. There are many solutions, just google around a bit.
It's a Mavic 700c Brake Post Adapter @Gaston. Been looking for one for a couple of years with no luck as they are no longer in production. Xtracycle produced some as well. There are some BMX brake adapters available that might work and other mods you can do. ACS made a cantilever brake adapter for 700c and 650b in the 80's, using MAFAC tooling after they bought them. There are many solutions, just google around a bit.