See Figures 1 and 2
This device is installed on 1970-72 models, to heat the manifold during engine warm-up. Engine coolant passes through openings in the manifold until the temperature reaches 150°F (65°C) exists. At this point, the thermostat closes off the flow of coolant through the manifold.
To test the thermostat, remove it and attach a length of rubber hose to the inlet end. Then immerse the thermostat in water heated to 175°F (79°C). After the thermostat has had a minute or so to reach the temperature of the water, force air at low pressure into the hose. If bubbles come out the other end of the thermostat, replace it.