Usually IC date codes are 4 digits, where the first pair of numbers specify the year and the second pair specifies the week. So without looking at any other references on the net, despite the ease of doing such, I would surmise that S9542 is the date code and that this IC was made in the 42nd week of 1995.
The prefix S might be used to bin the part according to one of the critical parameters (say, offset voltage or current), or to indicate a specific fab or facility, etc.