G'day Aaron,
Thanks for sharing your observation and knowledge.
Is the skirt the shiny brass exterior part that the old one obviously does not have, or a less obvious part? If guess 1 is correct, the new thermostat definately doesn't fit the housing, but there is a slight, close tolerance depression for the flange of the old thermo in the housing that means there is no way the water can go anywhere but through the thermo or to the radiator. The old thermostat is definately captive in the housing, with no way the coolant can leak past if the thermostat is closed.
Hope that makes sense,
Mark

