I would not be suspicious of the mag. Most police Lugers had their mags replaced one or more times during their service lives of up to over 30 years.
I would be very interested in knowing the serial number of this pistol, the exact grip strap marking, including all periods, numbers, etc. and whether or not the pistol has or had sear and mag safeties. If the marking is S.D.I. , it is from the Essen command of the Schutzpolizei of the Prussian administrative district of Düsseldorf (HWIS, Chap. 7).
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