What is CAPT & PURS?
It is a two position tumbler switch on the overhead panel to allow/disable the activation of the Evacuation alert from the cabin (explained here page 13). The view can also be found here for A320 and also here, with the following comments:


CAPT and PURS - the alert may be activated from either cockpit or cabin. 
CAPT - the alert may be activated from cockpit only.  


The A320 pictures seem containing the safety wire to keep the switch in the CAPT and PURS position, so it most likely another position is only selected for the reason. 

The manual says Allows activation of an Evacuation alert from
the cockpit or cabin but from explanation above and the notation on the panel looks like the alert activation from the cocpit cannot be disabled by this switch.

This electric component is also called "toggle switch" and may be called "flush switch". These components were patented as early as in 1917.