Verslagen, you are correct, the stock screws have a sealing washer. see pictures below. Stock screws with metal washers are on the left and the new replacement screws with the soft o-rings are on the right. Also note that the metal washers on the stock screws do not come off, they can move freely, but the can't be taken off the shaft.
I would be inclined to try the new screws with metal sealing washers but I figured since the stock screws with metal sealing washers were leaking, there is no point. The failure is probably with the petcock or tank hole. Does anyone agree?
I tried to get a pic of the underside of the tank too but it's difficult angle.