Ed, Lugers at Random pg 176."DOUBLE DATED DWM AND ERFURT'. He says "receiver dated 1920 or 1921 over original manuf. date of 1910 - 1918."
This is the only clue I could find. I was thinking that it had to be a re-work of an earlier military with unit marks.Or it is a Police marking. The only other thought was a fake. I have not seen the pistol.