It may just a cover for some type of service adjustment that is located back behind the hole, like the clutch adjustment or something similar. The hole may be so the tool can be inserted to make and adjustment. A mechanic should be able to tell you or a service manual. I doubt any fluids will need to be changed. If it was that important it would have a threaded plug instead of a rubber cover.
Looking at my shop manual, a 1980 1700 model, it says there is an access hole for tractors that have a dual clutch. The hole is for adjustment of the dual clutch. For tractors that do not have a dual clutch, the hole is there anyway. This is probably what your hole is. A tractor mechanic can verify this for you if I am correct.