If you hover your mouse over the C2693 or D2693 you should see a pop-up box that says "NEW SUNSHINE ROOF WITH LONGER MOVEMENT: DAT 15/03/1997" indicating what the change was, and when it happened. If you enter your chassis number (top right box. VIN) and then navigate to that page (Fiat » COUPÈ » PX COUPE' GAMMA'96 (1996-2000) » 701 MOBILE PARTS AND GLASS » 70122 -> FOLDING TOP » 70122/00 FOLDING TOP) then the correct part will be highlighted, and the incorrect part greyed out.
1996 Portofino 20vt & 2000 Pearl White Plus 1985½ & 2016 2017 Fiat 124 Spider + XF Sportbrake
The 16 sunroof has a shorter travel than the 20v (or at least later ones). From disassembling one many moons ago, the difference will be in the positions of the microswitches embedded in the plastics of the motor's gearbox.
Worst case, if you get the wrong one, the sunroof just won't open as far as it did (but a 16 with a 20 motor might run out of pushrod...) and you can probably swap the gearbox anyway.