you asked the question already in 2004 on Jan C. Still's forum, don't you?
The answer is: "Gewehr Richter" was a gun dealer located in Cottbus. The complete name is:
A. Richter, Gewehrfabr(ik), Cottbus.
I found two ads of Gewehr Richter in a German hunting magazine "Deutsche Jäger-Zeitung" No. 24 of December 1918 and the other in "Deutsche Jäger-Zeitung" No 15 of May 20, 1920.
By the way: I liked to know the full serial number to ad to my database. Should the SN be in the range 261000 to 261300 I would be even more interested in the full serial number. Or, if you could post it, a photo of the right side of the pistol.