Larry & Ron are both right of course. The pistol is not a rare enough model to seriously consider restoration. Besides, it's pretty cool the way it is!
Jerry Burney

A pistol with such fine workmanship may have the name or initials of the gunsmith concealed somewhere. The first place I'd look would be under the grips.

Sounds like you have the lightweight variations.

TAC,

Put a magazine in there and the hole will be filled up! :D

If the hole is just above and aft of the mag release, it may have been there for a police mag safety, that was removed, because they did not work. TH

Tom, I think TAC's British humor has escaped you this time... I suspect he was talking about the "hole" in the frame between the grips (magazine well) :D

The photos of this nickled Luger show no hole behind the grips, nor the small cutout on the flat side for the magazine safety to fit into...