I could be wrong, but I believe the round butts were reserved for short barrel guns to make them more concealable (under jackets and off-duty attire). So 2 1/2 and 3 inch barrels were the round butt guns. Of course, there are always exceptions, but they would be rare.
According to SCSW, the square butt was discontinued in "1995-96." This was in the middle of the -4 models. So all -5 models should be round butts, regardless of barrel length. -5 models should be easy to spot: They will have the frame mounted firing pin (instead of the hammer), but no internal lock system.