side-plate with cut-in on the upper side..

[John Sabato]

Welcome to the Lugerforum LarsB,

The sideplace you have seen with the cutout on the top side was designed to work with the Police sear safety.

I can't post an example photo at the moment, but the sear safety is a springloaded pin that is attached to that part of the receiver that covers the top of the sear. A protruding pin captures the sear throught the opening you have mentioned to prevent the discharge of a chambered round by pressing on the sear with the top half of the Luger removed.

When assembled to the frame, the sideplate lifts this pin out of the hole and permits the trigger function to release the sear.

I know this may be difficult to understand without a photo or graphic to explain, but I hope that my words have given you some idea what that cut is for...

If the Luger you saw it on does not have provision for the sear safety, then it was probably removed or lost, or the sideplate is not original to that particular Luger.