Dan,
The fact that the Liegnitz marking on the holster is canceled supports the sequence of ownership I postulated.
Your gun does have the police (sear) safety that Schupo illustrated and appears to have once had a magazine safety which was another safety added in 1934. The mag safeties were not successful and nearly all were removed. Your gun does not have a notch in the upper left corner of the left grip panel which would have been present with a mag safety. Either the photo is misleading or the grip has been replaced. In any event, the sear safety indicates the gun remained in police service in Köln throughout WWII.
I am confused about the descriptions of the pistol. At the outset, you stated that it is a 1913 Erfurt which would indicate that it has 1913 stamped on the top of the chamber, an Erfurt logo on the toggle train and many acceptance stamps on various parts of the gun. Without a photo of the top of the gun, I can't verify the first two but the available photos don't seem to show the acceptance stamps, possibly a consequence of the quality. Also, your wife's comment about a "42" stamped on the toggle train would suggest that it has been replaced by a WWII-era component.
If you really want to run this to the ground, I suggest borrowing a friend's camera and taking higher quality photos of the top, left and right sides, front of frame below barrel, front grip strap and rear grip strap. These should allow us to determine just what you have.
As to your question about a site on German WWII memorabilia, I suggest
http://www.wehrmacht-awards.com/.