Cleveland specific thermostats, with the hat, come in two styles: Standard and high flow. It is believed that originally our cars came with the high flow style. Both were readily available until recently when ROBERTSHAW discontinued the Cleveland specific version.
The standard flow version should still be available, perhaps with a bit of difficulty, from NAPA or other sources
It was the discontinuation of the high flow version that motivated pantera owner Marlin Jack, a retired machinist, to fabricate the hats that were then attached to the high flow thermostats, which were still available in the standard non-hat version. Last I heard he has ceased manufacturing the hats.
To fill the void, Tim Meyer, a machinist that specializes in Clevelands and is now manufacturing iron and aluminum Cleveland Blocks, Is now selling high flow Cleveland version thermostats that are duplicates of what had previously been discontinued.
Larry