This is good lesson in boat buying. Always check the tiltle against the hull number.
Before I retired from the FMP/FWC I worked alot of title fraud and altered HIN number investigations. It's a third degree felony to knowingly sell or purchase a boat with an altered HIN and or title in Florida. Florida leads the nation in stolen boats so you have to really check. In your case as an officer, during a stop the registration said 1979 and HIN said 1976 I would have seized the boat on the spot. You may or may not get the boat back depending what the title history shows. I've seen stolen boats sold two or three times to unsuspecting people and the guy who had it last is always out his money and might end up charged with theft. Be careful of the deal that sounds to good to be true... You guys replacing transoms it's your responsablity to be sure the HIN is placed back on the new one.
If your looking at a boat and the numbers don't match contact your local Marine Patrol or whoever investigates boat theft in your area or I can put you in touch with someone through IAMI, International Association of Marine Investigators.
http://www.iamimarine.org/iami/