A protective cap which has been designed for the YBO-BSB2, but can also be fitted to the previous model (YBO-BSB) and the Base Sounder (YBO-BS) when either of these devices isn't fitted with a sensor.  The cap is secured via a twist fit.
 
The cap can also be added to the isolator base, YBN-R/3(SCI) to create a standalone isolator.