Sounds like your WD45 has an external hydraulic pump on the front of the engine to give it "live" hydraulics with a reservoir behind the gas tank. The WD45 still has an internal hydraulic pump to operate the 3 point hitch arms and remote cylinder coupling - it's checked by the dipstick noted and takes the 20w hydraulic fluid mentioned. Then you have the transmission which is filled through the plugs .... simple enough?? ;-)
I have a few images of fill locations for the WD45 on my website under "oil capacities and oil type recommendations" - but none of them will show your external pump or reservior.