Switch box bench test
 

I tested my two old switch boxes but I don't know how to do it but what I did and maybe it will help you, I put the red lead from my multimeter to the top right stud and with the black lead I attached it to each other stud in the ohm feature all the studs had a number except one on top it said zero I did the same to the other switch box and all the studs produced a number. So I'm thinking that the one switch box was bad . I was just curious and was playing with them and did find one bad They are Mercury switch boxes maybe someone can use this info