X pkg is a snowcheck package Polaris use to offer when pre-season buying.
Special graphics, color matched bumpers, different snow flap & seat markings w/ an X.
The colors were unique from the standard offerings as well. Red/yellow, sonic blue/yellow 2002 & 03 & even black/yellow in 2003.
You could get an x pkg 550 fanner in 2003 for I had one pic is in my avatar.
It had nothing to do with the cylinders. Also pre-season order let you add different shock pkgs, seats, M10, EStart, & REV. A marketing scheme similiar to the graphics packages they offer now on the Shifts & Rushes.
Maybe your friend was thinking XC (non-VES) or XCSP for VES (Variable Exhaust System). The XCSP has exhaust valves.