Thanks to all for comments. The witness mark appears to be destoyed on a barrel, and I guess that the barrel was replaced. Small parts are not strawed but blued.
But note that this is not a single PSE barrel Luger, it has a brother! I have found this thread with description of the same mark:
http://forum.lugerforum.com/showthread.php?t=4633
The serial number with a letter photo is here ...