The front case is open to the room and has nothing whatsoever to do with the combustion chamber..
Air for combustion is drawn into the combustion chamber via the fan which is situated behind the combustion cover to which is sealed from the front case.
The type of cistern will depend upon which type seal to be used.
If a porcelain cistern use the conical seal , if a thin plastic cistern then use the flat one.
The plastic washer should be used between back nut and cistern , disregard the rubber seal not used.