Do I have the correct bucket seats? 1971 Demon 340
My two cents :
My '71 Demon, not 340 but slant-six, has also the M6X9 code on the fender tag. Followed by the black color TX9 code.
My seats are original and black.
But...
On my broadcast sheet, M6X9 is the trim code. I think it's the code for the painted upper door frame
TX9 for black
5X8 for charcoal (340 only)
Your fender tag has the 5X8 code, so I suppose your original seats were black/charcoal to match the charcoal upper door frames.
But it's only a supposition... :)