Shimano's STEPS Speed sensor unit for disc rotor position. EW-SD300 3.45 mm diameter . E-tube cable - 1400mm. For chainstay mounting. Plugs directly into DU-EP800 drive unit.
Best Price: £19.99 from Merlin Cycles
Shimano STEPS EW-SS301 Speed Sensor For Disc Rotor – Black / 1400mm Reviews