A gauge block assembly for use in conjunction with the refilling of high pressure gaseous oxygen cylinders. The gauge block is used in an oxygen transfill header or manifold and includes a gas transfer control unit that allows rapid evacuation of the spent oxygen cylinder but that prevents rapid refilling of the oxygen cylinder with fresh oxygen. The gas transfer control unit includes a valve assembly having a restricted orifice therethrough. During the evacuation process, the check valve opens, bypassing the restricted orifice, to allow rapid evacuation of the oxygen cylinder at a first rate of flow. During the refilling process, the check valve closes. The incoming oxygen must flow through the restricted orifice at a second rate of flow less than the first rate of flow thereby regulating the flow of oxygen.