It has been my experience that the factory lacked consistency in marking SBT serial numbers. Many higher grade guns such as Crowns and Monograms have a "C" or "M" after the s/n with an "S" at the beginning. I would interpret this as "S" stands for single barrel at the beginning of the s/n However, I don't recall ever seeing a Specialty with a s/n that reads SXXXXS; they are usually marked SXXXX and most of the time no letter at all.