Most of those, at least up here, date from the 1920's and 1930's. They were originally coal fired and most were later converted to oil. Gravity is correct. They were a gravity hot air system. No blower, no moving parts in the original. A single return, strategically located and an octopus style supply pattern.
We have slowly removed them over the years up here. To the detriment of comfort in many cases. They weren't overly efficient but were extremely comfortable. The house seemed to fill up with warm air.
That one is in much better shape than most that I run into. Not a "Wall of Shame" in my book.