Thanks Steve,
there were heaps of Clevelands sold in falcons and fairlanes in the 70's, and I looked at the original 73 (very dog eared) parts book in the local ford dealer - there was no sign of a design that even attempted to close the lower port. The illustrated Triton one has the lower section (labelled 16.8mm) which sits above the seat when the stat is cold. As far as I can see it moves down and closes the port below when the stat is open. A bit like the sketch shown.
Please let me know if this is wrong but this is the first time in 21 years ownership that it hasn't run hot so it made a huge difference.
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