We publish a very useful document about Lugers, all accumulated from the knowledge shared here on this site.
You can download it from:
http://forum.lugerforum.com/showthread.php?t=13121
The right side of the receiver should have original acceptance and proof marks. I am not sure what "DWM 14 - 18 markings" are and would like to see a picture.
As John mentions, the factory location for serial number is on front of the frame in front of the trigger guard. You should also find the last digits stamped in other places on a military accepted Luger, and the first and last digits stamped on the trigger side plate. First digits stamped inside toward the bottom, and last digits on the outside.
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Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.