What is this?

That looks like a TRANE Direct Return Trap for a two-pipe steam system. It was used when the original boiler was coal-fired and didn't have electrical controls. It has a float and trip-switch inside, so if the pressure in the boiler ever increased, water would back out into the returns, closing a set of check valves, then rise into the chamber, lifting a valve to allow steam into it that would push the condensate back into the boiler.