This is the in-tank fuel strainer used on the: 

                                        1975-1976 CB400F, ....  the 1975-1976 CB500T,....   

ONLY used on these models with the factory petcock located on the left side of the gas tank.  It screws on to the top of your petcock, and in turn the complete unit threads on to your threaded nipple on the left side of the tank.  ......Please see my other auctions for more NEAT HONDA STUFF.....  To help you keep your Classic Honda on the road.....