Well, I looked again and found it this time in the TM under Cab, "cab drain plugs".
The parts TM for the M1083, TM-9-2320-366-24P-1, figure 267, #3. Part #12417562, "VALVE,CHECK,CAB VEN".
Not really sure what it does, except plugs the access hole.
Hope it helps, some.
Tom.