Just to be clear about this and TC's info. In your picture, there are 3 rear lights. One oval on each rear body panel (sometimes referred to as fender) and one on the door with the license. When I was talking about "flat" lens, I was referring to the two body mounted lights being flat for 55-56 and stepped for 57-59. The 55-56 light on your rear door is flat and only on 55-56. The 57-59 door light changed to a bullet nose license light only, no red. The 1956 GMC accessories catalog shows several stop and marker lamps available. Mounted on front fender top, roof top and rear of truck. Several of these were meant for the big trucks and, as they say, "the safety minded person". Also a turn signal kit, with lights, was available. Don't have a Chevy catalog.
