This afternoon I rang the vendor which is a well reputed importer/gun shop situated not far from where I live, (where in these years I've already bought several of my collectibles)
He assured me again that the gun is "all original Krighoff" and he added that is not the first Krieghoff he's seen with what I called "heavy machinework" on the left ear above the safety.
Looking forward to tomorrow morning when I'll go to see it properly.

Thanks again Eric.

Just been to see the gun
Honestly it looks a lot better than from the photo.
We disassembled it completely.
All the Luftwaffe and acceptance marks were at the right places. Good bore.
The grips are the original brown fine dimonds brown plastic grips Hermann Ritzmann code 8964.
It was partially restored and the magazine is not matching.

Could it be a Krieghoff top mounted on a reworked Erfurt grip frame?

In fact I had considered that but it's not, this morning we carefully examined the gun bit by bit even with Gibson's book on KRIEGHOFF Lugers. Not only that , the vendor lent me a copy of that book for my information.

You'll probably find it amusing but having seen its price on Amazon this afternoon I rushed to a scrivenery (copyst) to have if photocopied as it's a bit too expensive :-)