I have a '66 C-10, '66 K-10 and a '66 C-20 all equipped with the factory reverse lights. I have seen another version of the reverse lights on 66's. I is has a cup style plastic lense commonly found on stepside beds vs. the flat glass lense commonly found on fleetside beds that was stock. The cup style plastic lenses yellow and break since they protrude a few inches out from the bezel. The reverse light is an RPO (Regular Production Option). Much of this information can be found in the 1960-1966 Chev/GMC Truck Factory Assembly Instruction Manual.