My 55 first series 6500 came with a Heil hoist, your cylinder is your reservoir, the "plug" on the top side of the cylinder is your fill point and the two previous post are correct on the procedure. Also that "plug" is a vent, if its to the point of looking like a plug and not a vent, or if its the original, replace it with a new one. Otherwise you can use a spray solvent or a quick soak then blow it out with compressed air.
