Green’s Dictionary of Slang

Crail capon n.

[the local fishery trade of Crail, Fife]

a salt herring.

[UK] in Jamieson Etym. Dict. Scot. Lang.
[Scot]W. Tennant Anster Fair II xix 33: Each to his jaws / A good Crail’s capon holds, at which he tugs and gnaws.