Both S and SO stand for vertical gun in Italian. What is significant is the number which follows. I would say 'S0' would indicate newer naming scheme meaning newer therefore more expensive to buy firearm. From what I have seen the higher the number the more "hand touching" involved in the final product. High number means mucho hecho a mano. I understand why there is mucho confusion PB is to Italy what W.W. Greener was to England.