Hello,
I assume they are three cases of 75x350, if they are not cut down.
The first is the classical headstamp on cases for the Army ( Guerre ). At 12 o'clock the calibre 75 DEC ( 75 DE Campagne ), at 6 o'clock the manufacturer, the lot number, the year and the metal furnisher. PDPs is Pinchart Deny à Paris ( manufacturer very common ) 1848 L 17 : 1848th Lot of 1917, And I don't know who is P.
The second is a lot rarer. It is the same case ( if not cut down reglementary to 200mm ) but for the Navy. 75 M 97 G is for 75 Modèle 1897 Guerre ( the Army ), MAI is Manufacture d'Armes d'Issy-les-Moulineaux, 12 L 26 12th lot of 1926 and D the metal furnisher: Compagnie du cuivre et duralumin à Dives. It is curious because on cases for the Navy The metal supplier is omitted.
The third is a Belgian manufacture. 75GP = 75 à Grande Puissance in opposite of the 75x280 stamped 75 TR = 75 à Tir Rapide. I don't know who is the manufacturer C B.
Regards,
Dandebur.