Thats a fuel filter of a 1970 Volkswagen Bettle. It helps maintain octane purity and emissions too. Soon I will install another fuel filter for the carbon canister return line. Currently I am working on a customaized (can paint) oil catch a can to be installed betwen the PVC and the plenum.
NOTE: 06NOV2004 UPDATE
*** The fuel filter between the cylinder head and the manifold IT DOES makes a difference. The car idles smoother, way smoother and does not have piston knocking anymore. I do need to install the Oil Catch a Can, cause the fuel filter in one day of driving shows residue of oil!!!