A 1"X20 thread is a Unified National Extra Fine designation, same as 3/4" and 7/8" UNEF.

http://en.wikipedia.org/wiki/Unified_Thread_Standard

You'd ask for a 1" UNEF nut, and short of a bolt and nut specialty merchandiser, I doubt that you'll find one. A FLAPS or a hardware store definitely won't have it in stock.
Jerry


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