Have never seen a gasket used, usually a single square "o" ring seals it. I believe that one of the differences between a TRW and a Federal pump is the single mounting bolt in the back of the pump, they are in different places. Your mounting bracket may be able to accept both pumps but in practice you will only use one bolt depending on your pump. To seal the can , you should only need two seals- that small square one for the rear bolt and the large "O" ring at the front of the can.
