Meurthe-et-Moselle is a creation of the Treaty of Frankfurt(1871). It originally was two separate departments, Meurthe and Moselle.
Under the terms of the Treaty, the Germans annexed all of Moselle department except
https://en.wikipedia.org/wiki/Arrondissement_of_Briey,. And the north east third of Meurthe department. The French then decided to merge the remaining two pieces into a single department. That's why Meurthe-et-Moselle has that weird bit that sticks up. That part was in Moselle before the treaty and the mergers, the rest was in Meurthe.
The really unusual bit is that the French didn't undo the territorial changers after the 1918. They regained the rest of Moselle department, which now contained bits of what had Meurthe and Moselle departments, but left the departments as they were divided 1871 by the Germans.
The Germans could easily take the
https://en.wikipedia.org/wiki/Arrondissement_of_Briey and leave the rest of Meurth-et-Moselle. Which they should have done in 1871, since leaving that rump bit meant Moselle still had representation in the French chamber to harp on revanche.
The Arrondissement of Briey would be the bit with all the iron mines, and very little people. The rest of the department has little value, and too many Frenchmen.
From a border perspective, Briey also removes the French from having a border with Luxembourg, which might be a consideration.