The 408 models of speed/Temperature sensors feature removable modules to allow cleaning and inspection of the paddlewheel.  A blank plug is provided to insert into the hull fitting during this operation.  The module and plug have dual O-ring seals and are secured into the hull fitting with a stainless steel pin and ring.  An extra safety feature on Model 408P is a length of stainless steel chain to hold the module in place should it receive a blow from the outside