Hi guys, yes they are rather rare. I believe that a number of manufacturess fitted them in the mid 30's and they were also sold as after market items. They did them with single and twin lens and I believe that they had a long cable so that the bulb holder could be undone and used as an inspection lamp. I have attached this advert from the 30's which shows a single lens option (top row, third from the left). Wish they were still 3/6.