Pictures here:
http://forum.lugerforum.com/album.php?albumid=44
According to the marking page on this site the crown over an 'N' stamp means:
(
http://www.lugerforum.com/lugermarkings/proof-3.html )
28. German "Nitro" commercial proof. Found on Lugers produced after 1920. Receiver proof is upright
So, is that the correct time period?