amuller, the motor I tore down has a bit different thermostat cover. It does not have that passage across the top. However, I think I can answer the question, hopefully without getting into another discussion. Look at the pictures I posted earlier and note the flow that is not thermostat controlled. See where it comes up from the head, goes through a groove on the bottom of the cover plate, then back down the backside of the head?
OK, the cover on e-bay is used on a different motor (maybe 10hp???). On that one, the uncontrolled water comes up from the head, goes through the passage you are asking about, then down directly over the center of the thermostat. When the thermostat is closed, it is free flow for the uncontrolled water passage. But when the thermostat opens fully, the poppet rises and partially blocks the flow from that passage across the top. Because that flow is partially blocked, water pressure (and flow) from around the cylinders increases, which increases the cooling of the cylinders. (Something to consider when contemplating whether or not the ‘stat is needed)
Following is the 6hp ‘stat cover that I tore down
