Inherited 1918 Erfurt with police holster and two mags

[OBXmom]

Here are a few more photos. There was only one mark and no number under the left grip. The right grip has a mark and the number 90.
There are marks on almost every piece and plain.
I included a pic of the other magazine marking.
I also included another photo of the serial number and the marking underneath it. What is under the eagle? I haven't seen another like it but I haven't gotten my hands on any books yet.
I understand how guns were turned in and double dated and reissued, but this only has the one date of 1918. Does that mean it was originally issued to the police? Or was it manufactured and sat unused until the sear safety was added and then went to the police?
I also find it odd that all the other 90's are so clearly marked.