Photo taken with its glass cover removed. Around 1960 many utilities that were using these (older) photoelectric lighting controls went to the "tubeless" twistlock units which were far more compact and simpler. Since the tube-units had a physically different method of plugging into the fixture receptacle, adaptor kits were offered that made conversion to the new twistlocks simple to install. The latter were standardized in 1959 and the three-prong footprint of photoelectric lighting controls remains unchanged.