Norton's part number for this cap, originally made in aluminum, was 06-1920. It should fit all Norton front disc brake master cylinders through 1974, I believe. It has been unavailable for YEARS, having been superseded to a garish plastic version that doesn't fit very well.
This listing is for ONE cap, in gorgeously polished stainless steel. You'll won't find a better example anywhere else. Less expensive aluminum versions also available.