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.
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My seats are original and black.
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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... :)
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