The Gaslow Filler Blanking Cap fits the male 3/4"UNF Gaslow filler connection.It can be used to blank off the back of the fill point to prevent someone inadvertently trying to fill without a cylinder connected.It can also be used to blank off the filler inlet connection on the cylinder to prevent material entering the connection.Or it can blank of one of the inlets on a no.2 cylinder, to turn it into a no.1 cylinder, so it can be filled safely as a single cylinder.