According to my memory of a discussion on Jan Still's Forum a couple years ago, the concentric rings appear on the earliest blueprints for the P-04 Navy Luger. There is a 1917dated Navy pictured in Imperial Lugers with a matching, non-ring magazine (numbers run lengthwise), and I think Still mentions the later, non-ringed mags in the text (no time to find it right now).
--Dwight
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