Could be the 145mm case for 75S Schneider (Tank gun or fortification)
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It's a modified case 75 DEC (75mm DE Campagne M1897). This gun came after 1879 so this marking is not a year.
Two docs give different length for tank case : 144mm or 145mm.
Sorry I did not see the marking 15 on your picture. Yes, it's a bit early and no new markings for repurposed case (?)
But I don't know when this kind of gun were tested. The BS (Blockhaus Schneider) gun came in 1916.
I only have this entry for 144-145mm case.
This one in picture is supposed to be a tank case (144-145) :
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