No idea what provision the Boxford makes but sometimes there are small cut outs in the housing that enable one to use a punch to tap the shell gently, or better arrange a screw press setup. Otherwise it is best to make a drift that just fits in the housing bore up against the edge of the shell to give you something to tap or press. This does not need to be a full disc, just a diameter with the appropriate curved ends to lodge against the shell over a reasonable area. What a bodger will do is just use a punch and chip lumps out of the casting.