Paul, actually my stall was the 40 mph one. I hope my tendency to mix numbers didn't occur here.
My first Model IV stalled at 43 and I flew it for nine years so that number was pretty much verified as I piled on the hours - 900.
The ASI might not be accurate at low speeds, but I did check it with a water manometer when building and it followed the theoretical curve quite closely - image of the spreadsheet attached. If there is an error, I suspect it would be common across the fleet using the factory supplied pitot tube.
I find this thread very interesting and it brings to mind some more things I might want to try on my current airplane. Thanks to all who have contributed and hope for more ideas in the future.