I suspect your carbine could have had that cracked receiver its whole life. The rear part isn't pressure bearing (unless the front locking lugs sheared causing the bolt handle root to act as a safety lug) so it is unlikely to have happened from firing. 1915 was when Italyicon first joined WWI and they like everyone else were short arms. I wouldn't be surprised if they cut a couple corners in production to try and get rifles to the front.

Other than that cracked receiver, very nice carbine! Hope you enjoy it.