Don't know if they did it as far back as mid-80s, but manufacturers "hid" another set of numbers for theft detection; learned it from the car companies. I can't imagine stealing a 43-foot sailboat, but whatever. Anyhow, if it exists, you may call the manufacturer and ask. Alternatively, you could search around the interior. It would be stamped to the hull itself, not to anything installed like a cabinet, headliner, or even the matrix liner, so you're looking for unfinished fiberglass. Because it's for theft detection, it will not be obvious, so under a hatch, behind cushions, on a stringer in the bilge, next to water tanks, etc. I've seen them under the v-berth by a water tank and in anchor lockers, but again, in the hull itself.