Off the top of my head, I'd say the change came in the early '90s. The difference is that the FS model has a safety device incorporated to prevent the rear half of the slide from coming off in the event of a slide separation. It's designed to save the shhoter serious bodily harm. Basically, the left side of the hammer pin is enlarged, and and the circumference of the enlarged head fits into a corresponding cutout in the slide which allows restricts any excessive rearward motion of the slide. I believe the 92F can be, or at least at one time was modified by the facory to FS specs. The left side grip panel from a 92F will not fit an a 92FS without modification because of the necessary relief cut for the enlarged hammer pin is absent.