All four = German military receiver proof. Found on DWM Lugers 1914-1918

Thank you very much, it's been forever, but I finally got my answer. xD

I believe those are the proof marks from the Imperial German arsenal that made them. What other marks are there? You can PM them to me if you'd like. Just obscure or omit any serial numbers if you post them anywhere else.

The only other significant marking is a cursive lower-cased "h" under the barrel and in front of the pistol ahead of the trigger guard.

Also whats the deal about serial numbers? I heard about hiding them often and I don't get it. xD

If someone else sees your serial number, they can use that to file a stolen gun report with the ATF. If you kept good records of sale (As you always should!) it would be pretty easy to prove it a false report, but it's still more hassle than it's worth. Especially when it's this easy to prevent. Try covering it with a piece of paper and duct tape if you need to show the area, or use Gimp to erase the number.

I heard this could happen, but never once has anyone manage to even prove it's happened when called out on it.

Guess I could black it out for now, but it makes the photos look awkward. I don't really know how to use gimp and make it all natural, is the program even free to download?

Yeah. Gimp is free. You don't have to make the cover obvious; just enough to make it hard to guess. You can even leave the first few numbers visible, and obscure only the last two or three.
I suggest using blurring and swirling, and anything to make it impossible to read.