The corrosive resistant Increasing Bush or Extender is used to increase thread size when connecting differing hose diameters (male to female).
 
M BSPT EXT - FM BSPP INT 1/8"-1/4", 1/4"-1/4", 1/4"-3/8", 3/8"-3/8", 3/8"-1/2", 1/2"-1/2"