Bolts are threaded into the back plate of the fitting so there are no welds or bolt holes that can be potential points of leakage.  These fittings come standard without a gasket. (With the exception of the 4”) & require a single gasket that is installed on the inside of the tank.  Available gaskets are cross-linked polyethylene.  EPDM or Viton (4” fitting comes standard with a cross-linked polyethylene gasket).