In these two chapters, the things that mysteriously reappear are both connected to the convict that Pip had helped back in the earliest part of the book.
In Chapter 10, Pip is at an inn with Mr. Wopsle and a stranger. The stranger is having a drink of rum and water and he is stirring with a file, of all things. Pip believed it was Joe's file, the one he had used to file off the convict's leg irons.
Speaking of leg irons, that is what comes back in Chapter 16. Mrs. Joe is found lying on the floor where she has been beaten senseless. Under her is found a convict's leg iron that has been filed apart.
In Ch.10 it is the file and in Ch.16 it is the leg-iron.
In Ch.10 Pip and Joe go to the pub "the Jolly Bargemen." There they meet a stranger talking to Mr.Wopsle who
was a secret-looking man whom [Pip] had never seen before. His head was all on one side, and one of his eyes was half shut up, as if he were taking aim at something with an invisible gun.
The stranger ordered three glasses of rum for himself and Joe and Wopsle. When the drinks arrived the stranger in a pointed manner stirred his glass of rum and water not with a spoon but with a file to communicate to Pip that he was an acquaintance of Magwitch to whom he had given the same file:
he stirred it and he tasted it: not with a spoon that was brought to him, but with a file.
He did this so that nobody but I saw the file; and when he had done it he wiped the file and put it in a breast-pocket. I knew it to be Joe's file, and I knew that he knew my convict, the moment I saw the instrument. I sat gazing at him, spell-bound.
The stranger departs after having rewarded Pip with a shilling and two one pound notes.
In Ch.16 Mrs.Joe Gargery has been found knocked unconscious and the police have arrived at the crime scene to investigate . Joe and Pip have just returned from the pub. Every one is clueless as to the identity of the assailant. However, the instrument of the crime is lying close to Mrs. Joe:
But, there was one remarkable piece of evidence on the spot. 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.
Pip recognizes immediately that it is the same leg-iron of Magwitch's. He is bewildered but is certain that Magwitch would not have attacked his sister. He is convinced that it is either Orlick or Compeyson the other convict:
I believed the iron to be my convict's iron -- the iron I had seen and heard him filing at, on the marshes -- but my mind did not accuse him of having put it to its latest use. For, I believed one of two other persons to have become possessed of it, and to have turned it to this cruel account. Either Orlick, or the strange man who had shown me the file.