No worries, Zippy - that X could have been in error. I've seen R & RED & W & X over the years, but whether that communication was "bulletproof", who knows ? I recall seeing film of mounted tires coming down a chute to feed a car, but don't recall the specifics. I think everyone's heard about the spares literally being pitched into the trunks, so I'd suspect an error there could be possible.
