I'm not familiar with this particular model, but it sounds as though it might be a double gas-discharge tube type arrester. We used to have similar protective devices on overhead telephone lines.
Being connected on the load side of the C/B results in the breaker providing protection if the tube shorts out, but it will still shunt lightning discharges to ground because the surge of current is of very short duration -- The tube will fire and conduct the energy to ground long before a normal C/B has time to respond and trip.
