Will my 1968 MF12 has the same switch your showing from Stens obviously not original as the manual shows the same 4 pin switch your showing. Mine never did charge so maybe this had something to do with it. I too don't know what the D pole is for.
Metric maybe Eh? :smilewink:D is for DONE! Stops tractor when done using. :smilewink: L on most switches if for Load or Lights and runs those items when key is on. M poles are needed for Magneto equipped tractors and NOT battery ignition. It is a ground. These are little odder marked switches, not familiar with those, but would bet same as some others, just odd labels we don't usually see.