I would say it's not M. It's a curved W. And, W and R always go together. I have never seen one with R alone, or that W alone. They always go together, and always in this relative position. Most likely, it's a single stamp, not two stamps.
Please also see the following two examples, and note the M over something on the typical prewar instance -- something might be another stylised M, and the stylized M alone on a later postwar example.
Now, here is my question: does that "something" (assume it's a stylised M) also appear on Mauser made Luger?
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