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Got to looking at my Bryant manual more closely from the 1956 installation. If my boiler ever had this option it was removed at some point and replaced with the on/off only valves we all love. I'm still studying but elsewhere in the manual they call it a throttling device. It appears like it senses pressure and adjusts gas flow within some range. There is reference to installing different springs based on what pressure range you wish to be in.
Anyway, right about now I'm wishing it was still there.
Anyone ever see this before? Seems like residential steam units have actually lost ground with respect to control in the 60 years since this was produced. Anyway, since I am still running the very model pictured here it is clear that these folks knew what they were doing when they manufactured it.