The "Zella-Mehlis (Thur)" marking alone would indicate manufacture between 1929 and 1945. Post-war Walthers were made in Ulm-Donau.
In the 'Gun Room' column (page 22) of the May 1999 issue of Guns & Ammo, there is a query about a reader's heel-mag release Walther PP. They even include a photo of the reader's pistol. In the same issue, but in the column 'Strictly Handguns', there is another query about a reader's heel mag release, aluminum frame Walther PPK.
[This message has been edited by Daniel Watters (edited June 11, 1999).]