do you really want to put any money into that thing? It has to be 60 years old at least. Looks like the system was probably a steam or gravity conversion. At a minimum you have to re-locate the circulator and install either another circulator on the other return line or a couple zone valves on the feed lines along with either a circ relay or a transformer for the zone valves. You also need to be sure the piping does separate into two distinct loops. Personally, I would tear that beast out and replace it with something that gets better than 45% efficiency.