The Allen head is the drain as well as removing the rear cover if you choose to do that, and clean it out anything that might be in there after 40+ years.
The hex plug it to be left alone as it is a not a drain plug.
If you have a leak it is best to clean everything up good and then you can see where it is coming from.
No sense changing fluid only to have to drain it again to possibly address a leak.
Hytran or it's equivalent, has the viscosity of approx 30W oil that you use in the engine.
Edit:FWIW:
the filter can be removed with only the amount of fluid in it being lost.
One does not have to drain the fluid to replace just a filter.
Do put a catch can under it so as to not make a mess on ones floor.
You do not have a separate transmission per say you have a hydro which consists of a Hydraulic pump and a hydraulic motor, no gears involved.
The hydro is connected to the differential assy with a gear set, and of course the ring/pinion with axle spider gears as a normal differential has.
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