This question may have a Global Answer (Affects everything with a X behind it).
I have seen a large assortment of Part #'s that have X's Behind them like my 2218X RX I received along with some parts.
First off what is the difference between a 2218 and a 2218X RX?
Second what is the purpose of the X's Behind other #'s does X mean it is a Updated Version of the Part
Another great example is the Glow Plug 3232 and 3232X