Steve-
Chances are that you have an English race and a headtube that is either french, italian or JIS. Most likey not Italian since ISO/Italian are somewhat interchangeable. What type of frame is it? Could even be swiss. It all depends, but it would help to know your frame type.
As far as I know, there are no shims that will help with this as the space is so minute and I don't know that a shim would be trustworthy for a headset.
Hope that helps.
Sean Flores
San Francisco, CA via Cape Town, SA