Butler manufactured a range of lights post WW2 for British military vehicles including these side/marker lights which are new old stock. I picked this pair up in the 1980's, still with remains of storage grease, intending to use them on a project but never did and, fortunately, I held onto the lights. There must have been a large amount of these released from storage in the 1980's onto the surplus market as NOS as they could be picked up relatively easily and cheaply then. I believe that this style of light was used from around the late 1940's through the 1950's.