The square plate is the licence plate but which of the two numbers the sail number is, I am not sure, I would have to check, but I am sure one of the other measurers frequenting this forum will know.
Have you contacted the associations secretary, Sarah Burgess? She keeps records of all measured boats. I am sure she can get you a copy of the original measurement forms and maybe a list of previous owners.
And while you are at it, ask her for a new certificate in your name. It allows you to participate in class races provided the boat passes its annual buoyancy test. AFAIK a new certificate is free of charge after a change of ownership.