I have an exact duplicate to the marker light pictured. I acquired mine from SP #4416 at Bangle, CA (a little south of Dominguez Jct. sitting on the SP-PE San Pedro trackage) as it was on its way to one of the L.A. Harbor area scrappers. Strapacs' Steam Locomotive Compendium says the engine was Lima #7721 built 10-37, vacated 4-5-55 at Sacramento and sold for scrap to Purdy Co. 6-1-55 at Los Angeles. I took my photo of it and acquiired the lamp, one of a pair which I discovered in a steel box on top of the tender, on 6-4-55. I believe these markers were mainly for light movements of the engine or if it was pushing at the rear of a train. Mine came with a 15W, 34 volt frosted locomotive cab lightbulb, about 3 1/4 inches in length with a base exactly the same as a 110v lamp you would use in your home. The bulb is tapered, from being the same size as the base to a diameter 1 5/8 where it is then formed in a half dome at the top. The markers at the front of the locomotive were of a streamlined design, so our style of lamp was always used only on the tender.