They were for HydraLectric cylinders that started with the 66,77 and 88s. There was a cord that went to the cylinder. The cylinders had magnets and stops in them. You would hold a button,get the cylinder to the position where you wanted it to stop for a certain depth with the implement,then you'd let off the button and from then on,every time you lowered the implement,the cylinder would stop at the same depth.
My 1550 and 1850 still have the cords wrapped up and hung on the tractor. My 1600 have been cut off.