GM step beds during 1955-1966 are almost the same. They even use identical tailgates. Thus GM did not find it necessary to change the rear bumper stamping during these 12 years. However, there is one important difference which distinguishes the 1955-1959 from 1960-1966 rear stepbed bumpers.
During 1960-1966 GM placed two stamped square holes (not in 1955 through 1959) on either side of the center dip below the license plate. This is because the later series had their license plate bracket attached to this bumper, not to the rear bed cross sill as in the earlier 1955-1959