1. Date of first registration
2. Date of first registration in the UK.
When the bike is exported out of Japan, the exporter is required (usually) to provide a dating certificate, the cert states the year the bike was manufactured, more likely though they use the date when first registered in Japan.
Mine came with such a certificate. Its that date (year) which the DVLA uses for 'date of first registration'.
Phil, its quite possible your bike wasn't registered in Japan until 1997, even though it was produced in 1996. God only knows how Fontyyy's MC28's were registered in Japan in 1993, a year before the model even started in production
grey as grey can be 
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30