I've never found one on my Halcyon 27 either. Maybe Offshore Yachts didn't mark the hulls in any way? E-mail Mark Johnson at the OYCOA class assocation...he has the archives and a lot of other info that might answer your question.
"To the best of my knowlege, H23s did not have hull numbers stamped/written/moulded anywhere. The only chance would be the original invoice from Offshore as that MAY be written.
Having surveyed every inch of FS I couldn't find anything "readable" as black mould had crept into most lockers. There MAY have been something written in pencil in the aft cockpit locker, but it was totally unreadable"