The holdopen is inside the gun and is operated by the follower button on the magazine...
When the last shot is fired, Or if you open the gun with an empty magazine in it, the hold open is pressed up and catches a notch on the underside of the breechbolt...
The holdopen became standard on the Luger but was not on the early models... many were refitted to include the holdopen including yours.
This photo cropped from your larger photos shows where the frame was drilled and the pin inserted on which the holdopen pivots...
Here is a shot of the inside of a Luger and where the holdopen sits by the top of the magazine well...
Take the top off your Luger and insert an empty magazine slowly and when the magazine seats you will see the holdopen pressed to the "UP" position ... You can also see the notch on the underside of the bolt with the top half removed.