world war one - What was this 1917 shell casing used for

Bought this shell from an antique store. Would like to know the history behind it, as in what it was used for? (The casing is about 16 and 1/2 inches in length)

world war one - What was this 1917 shell casing used for? - History Stack Exchange

Large calibre markings (The Western Front Museum)

Original France WWI French “Souvenir de la Guerre - 1917” Austrian Ber – International Military Antiques