unique HW radiator fitting

murph50

A friend showed me a picture of a single lower radiator fitting with both supply and return connections. The pipes enter on a horizontal 45 to the fitting. The radiators are circa 1900 cast iron columnar. Never ran across one before and was curious how it worked, ie. first thougt is that it would short circuit and not feed the rad. properly. So is there something internal to the fitting that can't be seen until removal that makes it work. The pipes in the picture both appear to be 3/4, nothing is known about the system other than the details in picture.