The E/N & E/J stamps you ask about are commercial proofs as described by the 1939 Proof Laws and applied beginning in 1940 according to other members. They are usually found on reworked guns of that period. I have a reworked Luger by Frankenschloss, Suhl that have the same proofs. Mine is in 7.65mm. I would be surprised if they were used after 1945 although anything is possible in that time period.
I have very similar proofs on my Anschutz rework. It has a DWM toggle, undated chamber notched for the artillery sight clearance and is in 9mm. I believe these reworks were assembled from existing parts or made from salvaged pieces ("Frankenlugers"). Of course this is a belief only, based on Kenyon's Lugers At Random and the last page of AArron's little book on IDing Lugers. These were supposedly made around 1940, but again, there is no proof of this.
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