The "M" code was the engine code for the tri-powered carb offered in those years. Although it was popular to order this option with the Sports Roadsters offered in 62 & 63, I think you could also order the "M" code engine on a regular Tbird. Therefore some regular convertibles and hardtops have the "M" code too. And some Sports Roadsters that were ordered without the "M" code have the normal "Z" code 390 engine.
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