Green’s Dictionary of Slang

Gourock ham n.

[Gourock, on the Clyde 40km (25 miles) from Glasgow, was once a fishing port]

a salt herring.

[UK]Hotten Dict. of Modern Sl. etc. 46: GOUCROCK [sic] HAM, salt herrings. Scotch.
[UK]Hotten Sl. Dict. [as cit. 1859].
[UK]Newcastle Courant 10 Apr. 3/6: ‘Gourock hams’ or ‘Norfolk capons’ are red herrings.
Lancet 2 813/1: The red herring is variously known in popular language as a ‘Glasgow magistrate,’ a ‘Gourock ham,’ or a ‘Norfolk capon’.
[UK]Farmer & Henley Sl. and Its Analogues.
[US]N. Platte Semi-wkly Trib. (NE) 14 May 3/3: ‘Digby chickens,’ ‘Glasgow magistrates,’ ‘Gourock hams,’ ‘Dunbar wethers,’ ‘Norfolk capons’ all meaning red herrings.
[US]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’.
[UK]Nottingham Eve. Post 22 Sept. 3/4: Among the dwellers on the Clyde a salt herring is called a ‘Gourock ham’.
[UK]Exeter & Plymouth Gaz. 24 Dec. 4/2: Herrings served as ‘Norfolk capons,’ ‘Digby chickens,’ ‘Dunbar wethers,’ or ‘Gowrock hams’.