Not totally relevant, but . . .
a friend is restoring a 41 Diamond T truck. His MANUAL instructs that the pumpkin (differential) is drained through a bolt hole at the bottom, and filled through a bolt hole at the top. In otherwords, unscrew on of the 12 or so bolts at the bottom, drain the oil, put bolt back, pull a bolt at top, fill from there, replace bolt.
Not sure why they even BOTHER putting a "drain plug" on it. The manual says the correct level is ABOVE THE PLUG, anyway. Weird.
Michael