Well, it is because the features of the upper are...unique to say the least. As you mention, all the numbers do appear to match. However, the crown N and "Made in Germany" stamped on the barrel, as well as the blank chamber, indicate that much of the upper was commercial instead of military production. The toggle train is Erfurt Production, and Erfurt did not make commercial Lugers, only guns for the military. Your sideplate also appears to be Erfurt military, and the top of the chamber has an "artillery cut" for an LP-08-which had numerous manufacturers. I dont think I have ever seen a Luger configuration like this.
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-QM
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