I wouldn't. The "M" stamp is over-rated. I've owned several 586 and 686 "no dashes" without the "M" stamp and never had a problem.
Somebody else will post saying that they did have a problem.
You have to decide for yourself. For "collector" purposes, I'd leave it factory original, but that's just me. Others will have a different opinion.
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'Merica!
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