The unit on the right is a Shuttle Pro video editing pendant. There's a plugin for Mach3 that lets you assign functions to the buttons and let's you jog the machine with the dial(s).
The plate is used to zero the Z axis, by jogging the tool down to it until it completes a circuit, and sends a signal to Mach3. There are macros available to automate the process.
Another Aussie Auto Tool Zero Setter
XYZ Probe modification