The class rules need to state the procedure for measurement, and what the legal values are. If they don't, then the class rules are lacking.
Most of the class rules I have read refer this back to the ISAF Measurement Instructions.
As for providing the measurements.... I think Prindle did this with the exception of the 18-2, and allowed for any sail maker to make sails... Like Hobie and NACRA; Prindle didn't provide the measurements 18-2 as again "legal" class sails were from them or a licenses/authorized sail maker..
