Far more were converted after the fact than were shipped from the factory as SAO. You are correct that the only way to know is to get a factory letter (which can take months), but I'd lean towards it being a conversion. The aftermarket stocks may be cool, but they reduce the value. Factory targets of that vintage can fetch $350 all on their own. These days $750 doesn't seem out of line, but I don't consider it any kind of super deal.
My '56 mdl. 14, which was my dad's carry gun during his deputy sheriff days, is the sweetest, most accurate shooter I have.