Nice one, close a door and open a window n all that eh
wet ball for old style (normal) caravan hitch or a trailer hitch, dry ball is for hitch with an anti snaking or stabilizer device that CLAMPS to the ball so the ball cannot be greased and must be free of paint and rust etc., there are friction pads in the hitch on the van that press onto the ball when a lever is pressed down, if they get contaminated they need replacing, hope this is clear enough