i highly doubt that, that there 69 GTX is an "M" code car. The Six Pack came out in 69 1/2 for the Super Bee and Road Runner. In 1970 is when they came on Coronet R/Ts, Super Bees, Challenger R/Ts, Charger R/Ts, Superbirds, Cuda's, Road runners, GTXs, and very few Sport Furys.
http://www.allpar.com/mopar/440.html