John,
I have seen here in Europe about 15 ww1 navy lugers. All but one had mags with concentric circles, one (1917) had a mag with a plain wooden bottom. This mag had a matching # (longside placed), and a C/M navy acceptance stamp.
In Jan Still's book Imperial Lugers, page 178, you can find a pic of a simular fitted navy Luger. On page 187 Jan explains that as from the second variation of mags most had plain wood bottoms.