another new, and my first luger

[Patrick Mc Mahon]

I recently bought a DWM 1918/1920 military luger. It has been reworked, as there is a small s/42 on the toggle, running the length of the toggle. It appears elsewhere (I am at work, and forgot to bring my notes, and somehow I am not sure looking at a luger while at my desk, will help my job!)
What I can not fahtom is just forward of the dates, on top of the frame behind the barrel, is a smll cut out, sort of coin slotted, and appears well done. At Joe Salters web site, there is a police rework luger with the same cut out, (#9230). This leads me to believe that the cut out was done by the German government, but why?
So far, my only therory is this is a cut out to fit the artillery luger rear sight. (not a good one, but I am desperate!)

any one know about this slot?

There is also some markings on the area by the lanyard loop- if I remember right, they are 9 and 33. I have not found any info on these yet.
Sorry if this is old news, but I am just getting started.
Patrick