If you look at the front of the block where the pump mounts, you will see, besides the main water transfer holes, an extra hole which only goes into the block a short distance, then turns up through the head gasket and into the head water jacket. This circulated water just through the engine and head when the thermostat was closed.
This passage gets plugged solid on some of the engines I've torn down.
Not too sure the older flapper valve thermostat sealed closed tight enough to be an issue of no built in by pass.
