I have a used Aristo/Zone VI cold light head (round type) that fits into my Beseler 4x5 enlarger. I am starting to play with some graded paper again after a long hiatus, and pulled this head out of storage for the first time in about 5 years. I'd bought this on Ebay and never used it. At any rate, I know what the two plugs do, one for the coldlight itself, and one for the thermo connection to keep the lamp warm. What I do not know is what the round plug outlet is for? There is a small round, 4 prong DIN type plug and I have no idea what it's for. Anyone have any ideas?