Adam,
Sear and mag (magazine) safeties were devices added to police Lugers in the 1930s. Here is a photo showing both:
The sear safety is a narrow strip of spring metal attached to the top rail of the receiver by a small rivet.
Most mag safeties have been removed or disabled. This one was disabled by clipping the part that blocked the trigger. Evidence of a mag safety includes the slot cut in the frame and the removal of the upper left corner of the left grip.
Does yours have either of these? I'll bet it has/had at least a sear safety.