RE: The writing on the fin
The serial numbers on more recent fins are made from a combination of the date of manufacture plus the actual consequetive number of fins made of that model. On the earlier fins, the serial numbers were made up differently and included some letters too. The letter actually indicates the brand group (S or ST for Starboard). The system has changed over the years but the serial never indicated anything about the fin's flex characteristics.
Hope this helps in some ways.