[DonVoigt]

As Ed says, I'll add that the receiver is from an artillery luger.
The proof marks of E/N and E/J are according to the German law proof law of 1940.
So this re-work/re-build of a WWI era luger was done after 1940 and before 1945.

Large and small factories were doing these "re-works" and the circle on the right of the receiver is likely a trademark or maker/seller mark from the 1940 era(JMHO).

Vono and Anchutz were large concerns doing this- but this is not their mark.
It IS Luneschloss; (but I do not recognize the mark and have not seen it before).
Update: 8-18-2018, Klaus says it is Franken and Luneschloss, see post below!

I do not believe it is a British/English proof mark, as it is not the correct size, style or location- though it could be a retailer mark- and would have to post date the 1940 time frame of the pistol if English- later than 1946.