According to a forum post, “ST” stands for a steno nib, i.e. a nib for shorthand. Under light pressure it writes finely, but it is considerably more flexible than an ordinary F nib: with a little more pressure the tines open and lay down a broader line, so that the fine differences between thin and heavy strokes in shorthand can be rendered (forum post of 14 August 2010).
On the Carrera, Montblanc's school-pen line from 1971 onwards, the steno nib was fitted only rarely according to the same post; most pens carry ordinary nib widths. Anyone who finds an “ST” nib too fine and swaps it for a broader one gives up precisely the quality it was meant for – it only shows its strength when writing with varying pressure.
The same abbreviation also appears on other Montblanc pens: on the 32 of the 1960s “St” likewise denotes the steno nib and not a steel nib – the versions with a steel nib were called 31 there. What “ST” means at Pelikan is answered by the entry on ST nibs at Pelikan.