The rear stepside fenders on the 1955 through 1966 are usually considered the same. The reproductions on both metal and fiberglass are listed in catalogs and related advertisements as being identical on the right and identical on the left.
Not true if you are a perfectionist! In 1966 Federal automotive regulation required all cars and light trucks to have back-up lights. Thus, GM modified this fender with a stamped indention to better fit the required back-up light assembly.