The FS has an enlarged hammer pin that fits into a groove on the underside of the slide. The main purpose is to stop the slide from flying off the frame to the rear if it cracks. This was in response to reported defective slides during U.S. Military testing.Beretta modified the model 92SB slightly to create the 92SB-F (the "F" added to denote entry of the model in U.S. Government federal testing) and, later, the 92G for French Government testing, by making the following changesesign of all the parts to make them 100% interchangeable to simplify maintenance for large government organizations.Modified the front of the trigger guard so that one could use finger support for easier aiming.Recurved the forward base of the grip to aid aiming.Hard chromed the barrel bore to protect it from corrosion and to reduce wear.New surface coating on the slide called Bruniton, which allegedly provides better corrosion resistance than the previous plain blued finish. Accessory Rail on bottom of frame by trigger guard.
Wikipedia or Beretta history.com has more on this, but I couldn't find anything specifically for the PT92 hope it helps