I believe 440s stopped the letter numbering after 75. Then they had the year stamped in the pad. I may be wrong on that but I think so. The "M" code referred to normally is what Tom said. A special engine. The most common use is the 69 440 Darts and Barracuda (1st real Cuda). 440s were available in higher C bodies. It isnt a far stretch to think they could be ordered for a lower trim level car.