kymbo is correct
all GTS25t (turbo r33 gts skylines) are typeM's
all GTS/GTS25 (non turbo r33 gts skylines) are typeS's
cruise control (rare) and abs were only an option on all r33's, so were airbags for pre 95 cars, all series2's have dual airbags std, and 95 series1's have at least drivers side airbag std. should definitely sell at $21k, give it some more time.
no such thing as an r33 m-spec, people must get it mixed up with typeM. the only m-spec is the r34 gtr nur/m-spec .