The ever-unreliable Preger (see Beuckels, Willem), Australian Hollandophile and author of The Humble Dutch Herring (Melbourne, 1944) claimed that herrings were used in the identification of German spies who might have been pretending to be Dutch:

Only recently an escapee from Holland who, because of his unusual accent, aroused suspicion, was tested by being given a herring to eat by the Netherlands authorities in London who were interrogating him. All suspicion vanished when, with three expert movements, he filleted the herring in true Dutch style.

He describes the action as that of grasping the two ends of the salted herring’s tail between the finger and thumb of each hand, then, flipping the herring over its own axis, a firm wrench would remove half the flesh from the bone…The other half would then be filleted with another firm movement.