The Stovebolt Site Forums The Shop Area Engine Shop Manifold Heat Valve

I think that two springs were used in combination as part of the heat-riser mechanism (the manifold heat control valve).

Part numbers (from the Master Parts Book):
3.640 Thermostat (exhaust manifold heat control valve)
3.653 Spring (manifold heat control valve)

I do not remember ever seeing both in place (but, my memory get worse every year).