The month and year of build are in the HIN number although I confess I don't know at what stage the number is allocated and applied.
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It sounds like one of the very last ones built at Marine Projects. In 1999 (I think) the moulds were passed to Prouts at Canvey Island who made a few more, branded 'Moody Excel 31', and almost all these were sold to the Med charter market.
I had an S31 for 12 years, I think she was listed as a 1997 boat but actually built in 1996, and there weren't many after her. Great choice of boat, and we'd still be sailing her had I not had a serious bout of bigger-boat-itis!