As @Raven (Elysian Angel) said, the S was the 9.13S (or the 9.12S, there was an option for the 9.12 to be upgraded with the S package except for the internal ECM) as an incremental update. Note that the standard 9.13 has the internal ECM in the enlarged spine. 9.13S focused on a new radar/WCS (mainly AA)
9.13S was dropped in favour of the 9.15 (29M) as a modernized variant with more comprensive updates (AA and AG), so only a dozen or so of the 9.13S entered in service. Due to the lack of funds in the nineties and the preeminence of Sukhoi in the "Halls of Power" in the Kremlin no "upgraded or modenized" variants were developed and all the founds went to the Sukhoi OKB and to the SU-27 family,