It looks to me its the cap, there was a plate that screws into the base plate and then the 'magnetic reader' screws into that. Measured the main rotar arm hight with the magnetic collar on it and with out and there seems to be only 1mm difference in hight. Cap seems tight on, gave the cap clamps a bit of a squeeze to make sure they are holding down. This latest kit was for the Ducelier so fitted it straight in.