The 73-80 fenders had side marker lights in them that match the front fenders. When the front went to horizontal in 81 the marker lights in the rear fenders were dropped & where Incorporated into the side of the tail light. This is the way I believe it is.
Of coarse, over the years MANY parts get swapped, fenders, whole beds, ect. so they may show up a different way than original...then factor in a guy with a welder while he is removing the trim, handles, ect. & they may go. If someone has more concrete info, other variations or if this info is incorrect...please speak up. Lorne