The big bolt ahead of the right hand gear shift is most likely the fill hole, can't tell were to check the oil level.
You need to contact a dealer or search Ebay for an operators manual and/or a service manual. You are going to need them at some point.
The operators manual will give you all kinds of information in regards to service intervals, filter locations and what oils to use where. A good investment for your investment in your new to you tractor.
Your transmission and rear differential are two separate reservoirs. The hydraulic system shares fluid from the rear differential reservoir. See item #35 on the attached parts diagram....Filler plug for rear differential/hydraulic fluid reservoir.. It looks like your's has been utilized as a hydraulic return port. Just pull the connection to add fluid.
See also the attached photo (compliments of UltradogMN) pointing to the square-headed pipe plug that you remove to check fluid level in the rear differential/hydraulic section.
If your tractor doesn't have these two features, post back.
OK, the attached photo (compliments of UltradogMN) illustrates the fluid level check plugs for a Ford 3000....front plug is for transmission, and rear plug is for rear end. Do you happen to have these (square-headed) check plugs??