Welcome to the forum. Never saw such mark on a Luger. Usually, German Polizei units marks, when applied, were located in the grip (rear or front). As Ronny, I'm looking forward to see more pictures of it and know more detaisl (caliber, marks in the toggle and chamber, proofs on the right side of the receiver - if any).
Valjean,
you are correct, the marking indicates property of the Reichs-Gendarmerie of the Reichs provinces of Alsace and Lorraine. It is believed that only about 400 of these pistole were ever issued with probably less than fifty known to have survived. They are highly valued by collectors here in the US.
Your pistol falls into the known range of R.G. pistols which are all commercial 1908 and 1914 models. Yours is a 1914 commercial model. I expect that it has a crown over "N" on the left side of the receiver. This is a commercial proof. In a few cases these pistols may also bear a gothic letter acceptance stamp. More photos would be nice.
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