It isn't 1/4" Whitworth... It is 1/4-20 UNC - a US screw thread.
As for Whitworth http://www.jag-lovers.org/xk-lovers/...th_system.html
One of the reasons that, as is often said of trying to keep a Jag running in the US, "All the parts that fall off are of the finest British manufacture."
And there are lots of WWII stories about trying to fix Sherman tanks and Jeeps with Whitworth fasteners.