I am remembering back 30 years to the 4020 with dual pto and JD110 answer sounds about the same. I thought though the older tractors had two separate stubs with a holder for the second stub mounted transversely over the pto housing. Could very well be you have one stub now that is reversible, just clean the exposed end before you insert it in the case. Dont remember any shift levers for the speed change either, it seems that swapping the stubs affected the speeds. Inserting the stub with the 21 spline gave you the 1000 rpm pto and inserting the 7 spline gave you the 540 rpm pto. Maybe a 5 minute job.