Vent on Radiator in Two Pipe System

tototwa

I have a two pipe steam boiler in a new to me house with 7 radiators on the first floor and one radiator on the second floor. None of the radiators has a vent except the one on the second floor. It has a Hoffman vent which is for a single pipe system. In addition there is a bolt on the radiator just below the vent which prevents the vent from being tightened and I think caused the vent to break at the threads when whoever put it there screwed it on. So it hisses and spits steam out of the threads. Overall the system seems to be working ok. However in addition to the steam and slight amount of water coming off the vent, it has started to clang a bit in the pipes going up to that radiator. If I turn that radiator off at its valve the clanging doesn't happen.
My questions are: Is there any legitimate reason for this vent to be there (everything I read says two pipe systems don't need a vent on the radiator)? Is it possibly related to the clanging? Should I take it off or replace it? How do I replace it with that bolt there? If I remove it what do I put in its place?

Steamhead

Take the vent off and put in a 1/8" pipe-thread plug.

Post a pic of the entire radiator, along with a view of the boiler area and any devices you find in the piping around the boiler. This will help us ID your system.

tototwa

The system has a main vent just above the boiler.