Okay BB, I'm heading to Charlotte to the NCGCA and Max Show. Here's what I think it is...
Modified property mark for the southern Berlin police district which would normally be S.B.S. These marks are according to 1922 Prussian police regulations. The only unit markings I have seen on the spine of a magazine were police markings.
It seems to me that such an abbreviated marking would be found on extra or arms room mags. This would explain the lack of a "1" or "2" one might to expect to find. The serial number, if there is one, would be meaningless as these mags would simply be spares left over from army use.
Like most German cities, Berlin experianced a lot of civil unrest and general chaos during the early 20's. I have see many photos of police units armed with every sort of weapon including MG08s, MP18s, C96s and light artillery pieces. In these circumstances imperial army equipment of all types might be found in police usage.
So there you go, that's my wild-*** guess. Klaus, Patrick, anyone else...step in and shoot it up.