A few days ago, we had a thread on GB stem bolt sizes. I finally located the two GB stems that I have, both are nutted types with GB embossed on the sides of the extension. One has the pinch bolt nut that has the raised round section, the other uses a chrome washer. I am reasonable sure that these have their original bolts and expanders and I suspect these are early-mid '70s vintage.
These stems do not use 8mm x 1.0mm fasteners. They use 9/32" x 26 TPI BSF Whitworth fasteners. I don't have an 8 x 1.0 bolt or nut handy to try the fit, but they are very close to that size and it's possible that the parts are interchangeable with a "class B" or "class C" fit (isn't that what Howard Sutherland used to call it?) I'm betting a tap or die could get one part to fit with a metric mate.
If you use a metric thread gage, a 1.0mm thread pitch almost fits. However, my very little used Whitworth 26G thread gage fits the thread perfectly. Another indicator is to try an 11mm wrench on the bolt head. If it seems just a little small, you probably have a Whitworth bolt.
Of course, this does not mean that GB didn't make stems with 8x1.0 fasteners at some point. They may even have made both metric and Whitworth versions at the same time, for different markets.
Steve Barner, glad that Whitworth has gone away, Bolton, Vermont