What's a CWS button?
Sounds like that plane has (or once had) an STEC autopilot installed - CWS stands for Control Wheel Steering - basically it allows you to set a pitch attitude / turn rate using the yoke, which the autopilot will then maintain until instructed to do otherwise.

If the autopilot is still installed you also normally use the CWS mode as part of the preflight routine to get the autopilot to activate so you can make sure you can overpower it (details on the preflight as well as using the CWS feature can be found in the autopilot manual, which should be with the POH).  

If the autopilot is no longer installed the switch technically should have been removed with the rest of the system and a blanking plate installed in its place.