The 1956 Chevrolet truck specification manual (at the link that I gave above) shows one standard rear lamp and optional dual rear lamps (RPO 249 - at rear of each body side panel).
Directional signals were in RPO 105/106 (1956 Task Force trucks)
Base on that GM information, it is possible that someone could have bought the truck without the RPO dual lamps and then added lamps (mounted anywhere in the rear) made by GM or by others.
