Yeah, I already did - I tried both my sets, and both sets of everybody else around me at the time who had metric/imperial ones... usually metric ones are too small in an imperial hole, and imperial ones are too big in a metric hole... in this case, the metric was too small AND the imperial was too big. Odd case of an in-between size!
In the end I adjusted it using a random key found in someone's toolbox, which belonged to neither metric or imperial! I think I'm gonna have to dremel one of mine down or else I'll never be able to chrono!
....unless someone tells me that there's another "standard" for allen keys that I haven't heard of.