Green’s Dictionary of Slang

Marblehead turkey n.

(US) a codfish.

1884		Staunton Spectator (VA) 24 June 4/1: ‘Bring me a dish of Marblehead tukey and look alive,’ repeated the customer.
1922		Eve. World (N.Y.) 13 Jan. 34/6: ‘Marblehead turkeys’ is a name sometimes facetiously given to cod-fish in Massachusetts.