I have a S 630 Satoh Bull Tractor 2 wheel drive ( 2 Cylinder Diesel). I am changing the Transmissio/Hydraulic oil and have a question. Right above the four bolt area that houses the hydraulic filter, there is a plug, which I assume is the drain plug???? It is approximateoly 4 inches above the filter area!

Close to the top of the transmission and on each side of the transmission there is a brass plug!
I have a service manual which indicates the transmission holds 3.17 gallons. In addition there is a note regarding the hydraulic system which states, "same as transmission".
My question is?? What is the plug right above the filter area for other than draining? What are the brass plugs for.
Next since the service manual says 3.17 gallons, I put in about three and one half gallons and I can't begin to see the oil??? How much oil does it hold??? Is the brass plugs designed as a filler plug, ( In short when it is full, does it drain out of the brass plug????
Any help would be appreciated. Thanks!! bandgatrifle@cs.com