I’ve seen a lot of confusion over the “S” designation too. For the longest time, the “S” versions—like the iPhone 4S or 5S—have essentially been seen as iterative improvements with refined internals rather than major design overhauls. You’re not likely to find a separate “iPhone S” model beyond that versioning system. If you’re looking for more detailed breakdowns, sites like AnandTech or Ars Technica have good articles on comparing the specs and design changes across models.