Being that's a dealership invoice & not factory, I'm thinking "Special Package" probably refers to White Hat Special/Spring Special.
No mention of vinyl top but Spring Special Plymouths were available with or without that.
Note how late that happened; the '70s were likely showing up on the lot.
As for the X = black sidewalls, I don't recall any BSW F70s or G70s.
For that matter, has it been established that the letters were scribbled on the lids at the same time as the tire size ?
Could they have been more like an "OK" (inspection) mark ?
Mine have looked "same time - same guy" but that's been an assumption of mine.