Charlie_Watt. Here is the battery box for the JTVs (I think): http://www.fullcompass.com/product/445858.html
Line 6 support told me full compass is where I should go for some things.
To add to this discussion (several months later): the pictures of the battery compartment here indicate all 3 contacts are in use, and go to a small PCB that has enough circuitry to indicate the 3rd pin is of strict importance.