Complex sentence: USAAF Modification Centers were World War II industrial plants "built to fit the mass production aircraft models to the needs of the specific theaters of operations" and were the only "set of field installations" constructed for Material Command.
Split into simpler sentences: USAAF Modification Centers were World War II industrial plants at which newly-produced aircraft underwent modification to meet the specific requirements of the theatres they were destined for. They were the only "set of field installations" constructed for Material Command.