My early '65 S1 1600 has a steel "T" device near the firewall with three hose connections. The later models do not have this. But I am not near my car to take pics or look and describe it. Perhaps someone else with an S1 can answer?
The pictures are not very clear in my manual but it does appear that I am supposed to have a "T" for the return hose so that the lower manifold connection will "T" into the return hose for the heater core to water pump.
Now I just need to find the right size T. Hopefully the hardware store will have a brass T that will work.