The top 'port' is a reverse rotation engine speed pto for I think irrigation.
The dual speed pto's have a shifter on the lower right of the rear case cover with a setscrew screw bolt in the bottom.
To make a dual speed pto you change the center hub in the lower gears, add the missing lower gear (top is in there) and install a shifter. You also need a 1000 rpm shaft. I changed a 1950T like this.
I am assuming that there isn't some oddball single speed unit like an 1800 might have.
RT (my 2 cents)