Chris--
1) My first thought also was that the tall thing is most likely a vent.
2) The Massey-Ferguson GC2310 is made by Iseki; the plastic cap with the 4 bumps on it is a fill plug. There are metal drain plugs on the bottoms of both sides of the front axle, but just the one fill plug on the left. It's supposed to be removable by hand, which my axle plug does once it's loosened, but the same plug on my HST fill tube must have bunged-up threads; I have to use a set of channel-locks to remove it- it fights all the way off. (Just a warning- it may be really tight!)
3) M-F recommends using the same HST transmission hydraulic oil in the front axle on the 2310, though there's been quite a bit of discussion on this forum as to whether or not regular gear oil is more suitable.
4) For the 2310, M-F says to fill so oil covers half the diameter of the shaft.
Hope this may be of some help--