As far as I know, as said above, the M is for Métal, which refers to the material of the case. I do not think it is related to the import duties and it can be seen in watches with Swiss case. Typically, I do not remember I have seen a mark (O for gold) in gold French cased Omegas.
But I do not know the reason of these marks! I can think of several hypotheses: an internal mark to distinguish between series of movements, maybe (but I am not convinced!) in relation to some quality degrees (the best movements for the gold cases); a way to get more control, thanks to movements, about the use of gold in watches. But I do not have any basis to say that!
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