At least up to 1996, HRC offered two 18" wheels for the NX5, one was 4.5" wide, the other 4.75" wide.
I can appreciate your pain in getting the wrong rim, bummer.
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30