oops, my bad.  I put a 3-way into a Custom 24 , replacing the 5-way (which I do not like).  I had to enlarge.  You won't.  
It's a five click rotary knob.  I call it a switch.  Eitehr way, you can get them from PRS. 
Matchless and Bad Cat use something similar on their amps.  Essentially a 5 position tone selector.