Whitworth history lesson...

...interesting tid bits via sohc.net and HondanutRider

An interesting trivia about Whitworth threads is that their pitch is Metric. While the profiles of the thread might be slightly different on standard Metric thread, they should match up and work if the same pitch is used. Whitworth nuts and bolts used unique head sizes that are different from either standard SAE of Metric sizes, and using a socket or wrench from these series will usually result in a "rounded" head.

Historically, Whitworth originated during/after WWI when the British brought some of the latest modern (at the time) thread cutting machines out of France for safe keeping during the war. These machines were capable of mass producing very accurate threads, but Metric of course, and the British war effort needed parts for the mechanized warfare that was evolving. After the war, they kept the machines and started producing their own "nuts and bolts" standardizing on the unique bolt heads. Because the original machines produced Metric threads, that's what Whitworth remained.

So, if a Whitworth bolt or nut gets "buggered" in removal. you likely can substitute a corresponding Metric-threaded one. However, it then won't be authentic and the "purist" concourse evaluator will mark you down.

...reprinted from sohc.net