Greg -

That little screw plug on the bottom left of the fridge, seals a drain
that goes straight to the fire box on the fridge.  It permits draining the
fire box of water in the event that water enters the fire box through the
vent (such as might happen when you are washing the car or during a
torrential rain). The rubber seal on the plug permits you to seal the
system which keeps the flame burning and the fridge operating.

Dave Crossman
'84 Westy - 280,000 miles