That's an interesting puzzle sorriaux, thanks for sharing
For me it looks like a k98 sight, put into German 1915 DWM, without receiver markings on right side but with a matching swiss TDL and magazine. And it doesn't end there.. some strange piece is placed behind the side plate (to improve mag release spring?) and the front of the barrel looks somewhat home made, making me lean towards a Schmeizer barrel made to fit the Luger. All probably done in a time where there was a lot more Lugers that people drawling over them..
..in short.. buy it!
(as a LP08 looking shooter)
Not all modifications is made to enhance the value. I'm guessing a similar situation in France as we have here in norway.. It's often hard to sell a rare Krieghoff for more than a S/42 in same condition. So putting that much work into 'faking' a Luger would be rather silly. Looks like this Luger has it's own special history to tell, and if you're lucky, it's pleasant to shoot with as well. Just my two pfennig
btw. does the receiver have the notch for the rear sight? ..over the chamber, towards the barrel.