All information comes from the book by Dieter Marschall.
According to the serial number, this Walther Model 4 is the fourth version, serial numbers range from 67700 to 223000, 7 or 9 serrations on the slide - the pistol has 7. Manufactured by Walther, as there is no mark of a license manufacture on the front left side of the trigger guard.
The marking N under the crown is the marking for the civilian proof test.
The year of manufacture was definitely before 1919, because from then on the places Zella-St. Blasii and Mehlis were merged to form Zella-Mehlis and the pistol is labeled with Zella St. Bl.
According to Dieter Marschall, the 4th version was partly labeled with Zella-Mehlis already in 1916, so that this pistol can probably be assumed by 1916 at the latest.