These lamps are not common although a few have turned up and there are two types. This one is the earlier one with a less than satisfactory pre heater. The later one has a pre heater cup under the generator instead of to one side as this one is. The patents for the lamp date from around 1920 to 1923 and the adverts we have date from about 1922 and 1923. I suspect that they were only made for a few years which is why there are few survivors.