Green’s Dictionary of Slang

a salt herring.

1859		Hotten Dict. of Modern Sl. etc. 46: GOUCROCK [sic] HAM, salt herrings. Scotch.
1864, 1867, 1870		Hotten Sl. Dict. [as cit. 1859].
1874		Newcastle Courant 10 Apr. 3/6: ‘Gourock hams’ or ‘Norfolk capons’ are red herrings.
1889		Lancet 2 813/1: The red herring is variously known in popular language as a ‘Glasgow magistrate,’ a ‘Gourock ham,’ or a ‘Norfolk capon’.
1890–1904		Farmer & Henley Sl. and Its Analogues.
1897		N. Platte Semi-wkly Trib. (NE) 14 May 3/3: ‘Digby chickens,’ ‘Glasgow magistrates,’ ‘Gourock hams,’ ‘Dunbar wethers,’ ‘Norfolk capons’ all meaning red herrings.
1901		Minneapolis Jrnl (MN) 13 Apr. 8/3: A herring is called in different localities in England a ‘Digby chicken,’ a ‘Norfolk capon,’ a ‘Dunbar wether,’ or a ‘Gourock ham’.
1913		Nottingham Eve. Post 22 Sept. 3/4: Among the dwellers on the Clyde a salt herring is called a ‘Gourock ham’.
1947		Exeter & Plymouth Gaz. 24 Dec. 4/2: Herrings served as ‘Norfolk capons,’ ‘Digby chickens,’ ‘Dunbar wethers,’ or ‘Gowrock hams’.