Len, I can take a look at the threads on mine, but I won't have access to bolts and thread gauges until next week. I have a memory of the thread being a standard one, but it's quite possible I'm thinking of something else.

I would be surprised to find an imperial thread, but you never know. German and other continental manufacturers did use UK-inspired fine-pitch small diameter threads (BA series in particular) much longer than seems logical. Larger threads are almost always metric, except for things like garden hoses and spark plugs - neither of which is usually found on a Norma.