hi all can anyone explain to me the small valve looking assembly on the 272 etc intake manifold under the left back area where the exhaust gases pass through. some manifolds have this bashed over and some dont??? i am not clever enough to work this out so all opinions welcome 

Sounds like You are referring to the opening where the Choke Heater Tube passes through the Manifold. If You look on the other Side of the Manifold there is another hole where the Tube goes in or it might be plugged also. The Tube rusts out inside the Manifold. If it is not plugged on the manifold or the Tube replaced a Vacuum Leak can occur. Some guys plug the openings and some replace the Tube and hook up the Pipe to the Choke. I have an Electric Choke on my newer Holley Carb. I plugged the Openings by Tapping and screwing a Bolt in to Seal it. Maybe someone will Post a Picture showing the Original Setup. The Tube through the Manifold can be replaced and is available as a Repro Part.