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The letter designation refers to the serial numbers. M series is the last model made before Rolex changed the watch designs a bit, going to ceramic bezels (more scratch resistant) and slightly wider case with wider crown guard on the Submariner.
These changes happened a few years ago, not sure of the exact year but I think 2009.
IMO the non-ceramic later model Subs like the Z and M series are great investments. They can be had for around 5K in mint used condition, and you'll always be able to get your money back, and probably more over time.
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