the trapeziod plastic plate has been there since the conception of the sg
its there on 1961 SG's, its there on 2012 SG's
but on some SG's over the years they have put the neck further into the body (as i suspect may be the case in 1989, certainly with the late 80's early 90's SG specials) and some they have increased the length of the fretboard for 24 frets (very 2012-13 special/studio). its always been there on 'classic' SG's, but not always on the cheaper ones. although other than that it certainly isnt a mark of quality