This old friend of mine is RS23L9A........

"F70" seems obvious enough.

What does the "X" refer to?

I've seen "Red" before. I can't remember if I've ever seen anything referring to a whitewall like "W",
but I have definitely never seen "X" before (other than my own car).

I've kicked it around with a couple people, some say it means the spare tire (and hence, all of them on the vehicle) were flipped around
to blackwall.

What I have been told sounds plausible, but....no idea. Anyone?

thanks