The differences between these two series of tail lights is an excellent example of lowering costs during production. To keep competitive, manufacturers will always consider making products of equivalent quality, but at lower prices.
In 1960-1966, GM, as well as several aftermarket companies, used a redesigned tail light lens and eliminated the need for the earlier metal bezel. The new plastic lens wrapped around the front edge of the same metal housing making it one piece. This new lens was created so it could also replace the previous 1955-1959 lens and bezel combination. Therefore, as supplies of 1955-1959 lenses were used up, dealership parts departments would offer the later style lens as a stepside replacement. This made the original 1955-1959 taillight the 1960-1966 type.
The black housing and wiring are the same from 1955 through 1966.