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The "M" stamp isn't for a barrel modification, per se - rather, it was a problem with the firing pin and bushing that, with some hot loads, allowed (sometimes, or not really, depending on who you talk to) the primer cup to extrude into the gap and lock up the cylinder. This was an issue with the "no dash" and "-1" models only, and also applies to the 581, 586, and 686. Sorry, don't have any good ideas about finding out more on the WRPD stamp.
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