Some of them had a pipe plug about 3/4 way up on the left side. (as you are setting in the seat)
It was a oil level and fill hole. I used a 90 degree street ell and small funnel. Housing would air lock often and took a long time too fill. Not user friendly at all to fill or add.
Also it's common for those to get lots of water in the xmission, Rain water runs down the shift stick. I've seen some with so much water that they would freeze and not shift in cold weather.
I've taken a small plastic cup and made an umbrella over the shift lever at the bottom of the shift rod and have the up side down soft plastic cup full of sealer so as to shed the water that runs down the shift rod lever.Since you now see the shifter area just fill through the shifter lever area. 80 to 140 differential oil ok. You can use Permatex sealer around the shifter top and or the gasket is easily made. Place in N when removing and careful when re-installing, make sure it's shifting ok after assembly and no engine use before attempting a drive test.
I got into some of them type axle seals that seal was leaking due to a bad axle bearing and/or the axle worn behind the seal due to axle itself worn in the bearing. Had to much up down slack for a new seal to seal properly.
If you see up down slack in the axle it requires pulling the unit, taking completely apart (splitting the case) to change the axle. (sometimes easier to just keep a eye and ear open for a old salvage JD and swap out the unit) Peerless/Tecumseh has a free Service manual and most of those are listed.
Several of the JD212, 110 and the old Sears SS 16 plus other used that type diffferential/xmission.
I only work on those for my own use, not for other people. (too much time and too costly.