Bill,
Look in Still's Weimar Lugers at pages 243 and 244. It doesn't answer the question you are looking for but does show another example. It also lists that when the book was printed, M10, M9, M26 and M28 were the only known examples. Your M81 is far from the grouping of the known examples.
With so few examples, the meaning may never be known.
William
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