Does the car have a "heater control valve"? It's a valve that regulates the flow of hot coolant into the heater core. My old Volvos had these valves, a lot of cars in the mid-60s had them, made by Ranco. On the Volvo, the valve mounted on the cowl on the passenger side under the hood. There was a thin copper line and a bulb (thermocouple) coming off the valve and ran right on to the heater core.
The idea is this: when you set the temp slider to a specific place/setting, the heater core heats up, and the bulb tells the valve how warm the core is, relative to the setting desired at the slider. The valve has a bi-metal spring inside which opens/closes to allow or constrict flow, in order to maintain the temperature you've selected. The valve has a range of...I forget...something like 5 degrees F. So when you set the slider, the valve keeps the temp where you set it, and if the temp drops below that setting, the valve opens for more hot coolant flow to the heater core. If the car is running warmer, the valve will close off the flow to the core. These Ranco valves do go bad, and there is one guy out there somewhere who actually rebuilds them. He's a crabby old crank to be sure.
The drawings show something they are calling a "Water valve assembly", which sounds suspiciously like a heater control valve. I imagine you could bypass the valve, which would bring hot coolant to the heater core 100% of the time, which probably wouldn't be good in the warm months. I did this on my Volvo, and when I used the heater it was fine, never go too warm in the car. In summer, I had a bypass set up underhood which bypassed the heater core altogether.
