The piston ridge helps determine the mixing flow inside the engine and control the flame pattern. You appear to have a K&B 40 there. The pictures ar so dark and blurred it's hard to tell much more. Based on K&B40's I owned in the past I'll say the following:

They used to put a rotary baffle in the exhaust port that closed off at low RPMs. One of the purposes was to keep up some backpressure at low RPMs to help keep the plug hot. An idlebar plug was often used also.
Back then muffler pressure often was not used and if it was it was tapped right at the exhaust port. There were some aftermarket mufflers for this engine that required removal of the baffle to install. I think Slimline made one.
http://www.mecoa.com/kb/index.htm for K&B information.