Sam,
X2 simply means rated to 250Vac, self-healing. The best of them are polypropylene. The purpose of a snubber cap is to prolong the life of the power switch, which would otherwise arc at switch off due to the inductive kickback from the twin toroids of the power supply.
I include these caps in the 100W kit, but not in the 55W, where the currents switched do not justify this extra component.
Cheers,
Hugh