Dunno if this will help you, topol-m. http://www.aviapedia.com/documentation/su-27-family-visual-differences-table
Off-set trackball seems to one of the ways to differentiate. That is between Su-27 and Su-27sm/Su-35.
Some talk about the adding of a second antenna on the back, that is with the sm3
Might be able ti find more here. http://s362974870.onlinehome.us/forums/air/index.php?showtopic=225995