To keep vehicles base price low, GM made many items dealer accessories. If the buyer required extras, the dealer was the installer. This created less complications on the assembly line and added more income for dealerships.
One of these extras was back-up lights on the 1960-1966 Fleetside pickup. After 35 years they have become very rare due to their location below the taillight assembly. They were always subjected to water and salt. Corrosion of the chrome outer die cast bezel is a normal result of trucks used regularly.
The adjacent photos show the light assembly before installation plus their correct location on the pickup box.