Hello Peter
I own a Gravely model D since I was born as my Grandpa bought her in 1939.Th oil pump is a problem as the original american models Dhad the pump inside the big plate, just close to the big wheel and, later the english made models D had the oil pump outside the crancase, protected by a little hood in the left side of the tractor. Do you understand what I mean?
Anyway the tractor needs only one pump, so don’t worry. Just watch if oil is turning back to the tank, taking off the cap and watching if oil is running from a higher pipe in the tank. If oil runs, no problem.
You can find infos about models D through the Gravely Tractor Club of America (manuals) or through the English Rural Museum in Reading University (contact Danielle Kristy Mills).
best regards
André