The guide rods 18,28 are shouldered as indicated at 46 (FIGS. 3, 4) and the sleeve brackets 20,32 are provided integral therewith locking means such as latches or snap rings or the like as indicated at 48 (FIGS. 3, 4), which are arranged to snap into the reduced diameter portions of the guide rods against the shoulders 46, for locking the extended assembly in thrust force bearing and gas-sealing c