Given that Leica has a long history of using 55° Whitworth (e.g. the 36mm Leica lens thread and their microscope lenses),
I'll accept that the standard tripod head screw originally used that threadform.
Except at the tightest tolerances, 20tpi UNC and 20tpi Whitworth are intermatable with no issues.
This is 2012! I can't believe that we're still discussing Whitworth threads.
This is interesting (if you like that sort of thing!).
From Wisner's website:
The base of the camera has a 1/4" and a 3/8" thread. I'm not sure which way round the American/European designation is.High strength bronze tripod plate with European and American threads.
Picture here: http://harrysproshop.com/Wisner/trad...osed_Front.jpg