The double S could be in a Johnson rough back with either brown buttons or smooth "baby dot" snaps. It was during the period that double "S" knives were produced that brown buttons were transitioning out. A grey combination stone with white print is most likely correct. The model (3-6) stamp should appear on the back of the sheath, but I have seen examples from this time period that do not have the model numbers stamped. Either way for you would be fine. Yes, the model 1 would be fine in a current sheath.