Return Vent

rbog110730

1922 Steam residential heat system. Part of house heats right up and part is slow to heat. There seem to be (2) returns to the boiler. The radiators that heat up feed the larger vent pictured (rounded top), you can also hear/feel air coming from it as steam enters system. The other picture is what looks to be a vent and it is at the end of the return for radiators not heating very well. Is this actually an air vent? How can you tell if it is working or not? It is much smaller than the working vent. Any info would be appreciated.