I've been researching these devices and actually they seem to have been in use from the 1920's through the late 1950's, although I found references that some were still in use as late as the early 1970's (especially in Britain and Canada)! The common "dose" would be 20 seconds or so although some machines could be set from 5 to 45 seconds with a timer; later, legislation made the limit 5 seconds. In the 40's, the American Standards Association established a “safe standard or tolerance dose,” that the feet receive no more than 2R per 5 second exposure. It seems that 60 percent of the tested machines were in the "safe range," although overall the range was found to be 7R to 14R per 20 second exposure. I haven't found any reports of injury to customers, but the doses were certainly higher than is considered safe now, and there were a couple of recorded operator injuries (burnt leg, dermatitis of the hand).