Green’s Dictionary of Slang

a thief who specializes in removing the contents of letters; thus poultry-rig n., the crime thus performed.

1811		Lex. Balatronicum n.p.: Poulterer. A person that guts letters; i.e. opens them and secretes the money. The kiddey was topped for the poultry rig; the young fellow was hanged for secreting a letter and taking out the contents.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue [as cit. 1811].
1859		Matsell Vocabulum 69: poulterer A fellow who opens letters, abstracts the money and then drops them back into the post-office box. ‘The kiddy was pulled for the poultry-rig,’ the boy was arrested for opening letters and robbing them.
1862		E. de la Bédollière Londres et les Anglais 316/2: poulterer, voleur qui ouvre les lettres pour extraire les valeurs qu’elles renferment.
1882		Dly Dispatch (Richmond, VA) 1 Nov. 3/3: A ‘poulterer’ is a fellow who opens letters, abstracts the monety, then drops them back into the letter-box.
1895		C. Crowe Aus. Sl. Dict. 61: Poulterer, one who steals money from letters.