The hull/sail numbers for Day Sailers were generally assigned to be the serial number of the boat as it came out of the mold. When the gov't. started requiring the numbers to be stamped into the transom, O'Day didn't start at 1. By the time they built the last class legal Day Sailer it was hull number 12943. The subsequent builder, Spindrift one designs, used 11000-11920(I own hull 11750). The next builder, Precision boat works, used numbers 13000 to 13113. After Precision stopped building the DS, McLaughlin boat works picked up production and built boats with hull numbers 13150-13165. After McLaughlin came Sunfish/Laser, who built the DS beginning with number 14000. SLI built around 150 boats. Now, Cape Cod builds the boats, they are beginning with 14201.
I gleaned this out of the Day Sailer Handbook for 1996. It's available when you join the class association, as is a directory of other class association members.
I've never heard of a class number as opposed to a hull number, but I don't see that as a big deal. If that sail is the original sail, the number's probably correct.
The class rules say is that the mainsail and spinnaker should have the "correct" sail number... But there's no definition of correct. Probably no big deal unless you come across a "Sea-Lawyer" at a sanctioned regatta.
Just go out and sail that boat!