Mrs. Joe was hit on the head with ______, which causes Pip to feel guilty again.

From the text:

 She had been struck with something blunt and heavy, on the head and spine; after the blows were dealt, something heavy had been thrown down at her with considerable violence, as she lay on her face. And on the ground beside her, when Joe picked her up, was a convict's leg-iron which had been filed asunder.