I thought the OP was referring to the extension on the plug.
It is indeed the gas plug. Fine threads on the cone shaped plug to prevent it from being screwed in too far.
The hole through the square head that extends out of the gas tube allows a tool in the butt compartment to be used to unscrew it for cleaning.
There is no adjustment of the gas system.
It is still my belief that the extension was used as a stacking rod. There is no other explanation for it being on there, at least that I have ever heard.