This is the cover used to close off the hole, for the lever attachment, of the main shifter, on older unimog gearboxes. You may have to tweak the overall size by +/- 1-2% depending on your printer, to get a good fit. It was used in the video "Unimog Barn Find. Part 12 - Let's start looking at that engine!" on the TeamArmyTrucks YouTube page.
Download from thingiverse