This is a custom made rack that is designed to be used with a crane. It has a removable section that has quick release pins to be able to use the crane and have it turn and not hit the rack. Many times a crane is only used once in a while, but the rack might be used quite often.
Also note that this rack is mounted on the inside top edge of the service body and this design rack can handle more weight because of where it is mounted being a very strong point on the body, but also due to the full length angle iron spreading the weight across the entire length of the body.