This missing horn is actually pretty common.
Processing large piece of metal is (or at least was at the time) a quite complicated problem: because of large dimensions you can have large temperature differential that may cause non-relieved inner stress while cooling.
The horn on anvil is a specialty feature, generally for curved items (eg horseshoes or larger pieces).
So when a blacksmith would break the horn, or the horn would break during manufacturing, at the time, they would not have thought of trashing the item or remelting it. Instead, they would simply sell the anvil to a less specialised profession, for example my grand father had one with the same defect. He was a carpenter and only needed it for small work like forging nails or metal fittings, so a "second" was enough (as opposed to a professional blacksmith that had all a variety of pieces to produce).