It’s marked as being made from a high explosive shell, so either it’s a shell case or else it’s a projectile. It couldn’t be a modified projectile, as it has no taper, and HE shells are made hollow, so you can’t turn them down to a cylinder. It’s clearly steel though, so not a case.
My best guess is that they took the hollow cast projectile, and then instead of turning it into a shell head, they simply turned it to a cylinder instead, then added a small door to the end.
