The diode plate is a spare part for the alt, should be at least available at the dealership of GM or at autoparts, automotive parts or NAPA parts, if they still have these old parts in the program.
Tecnically, the diode plate cuts down the alternating voltage of your altinator, which produces AC, half down to DC of the swelling type. You can not measure that, for your equipemt is not fast enough to follow the cycles or the half cycles.
In the alt the diode plate is that peace that has a lot of cooling fins and some contacts in the middle, three of its kind.
As far as I remeber from my last alt repair, the housing is fixed to the minus of the alt, with a M6 treat.
By the way, if you mix up 12 V, which is considered to be minus of the right side alt, and 24 volts, you produce big sparks with your batteries connected and after starting you make the alt smoke, which means you just killed ist, if it survived that long.
Wolf