Another shot at the checkered sideplate: Since the gun has been sporterized could it be that the owner wanted a better grip? If you shoot with two hands your left thumb could rest/press against the checkered part to help stabilize the gun.

Balder

Thank you for your offer Doc, but since the Luger is a DWM and that is a shooter, I prefe to have a wooden magazine. But I will take a K date magazine any day :D
Alf