That valve on top of the hydraulic pump is the "hold control" valve. It's operation and adjustment was complicated enough there was a decal on the side of the battery box showing the adjustments. Of course that is all in the operators manual too.
Of the three ports, the two across from one another will work for "hold position". The third port does not. Otherwise you can plumb off there any way you desire. AC recommended plumbing when adding a loader was to place a tee in the line on the right side, inside the frame, of the line which went to the right rockshaft cylinder. Then a coupler and lines fed the loader cylinders. So most anything is possible.