Sounds like MacDonald Fraser sourced it from a Phil-Alfie (Price)-Kaiser Bill incident in Devon. It’s fiction’s loss that GMF didn’t discover the Sausage Duel:
As a cofounder and member of the liberal party (Deutsche Fortschrittspartei) [Rudolf Virchow] was a leading political antagonist of Bismarck. He was opposed to Bismarck’s excessive military budget, which angered Bismarck sufficiently to challenge Virchow to a duel in 1865. Of the two versions, one has Virchow declining because he considered dueling an uncivilized way to solve a conflict. The second has passed into legend, but was well documented in the contemporary scientific literature. It has Virchow, having been the challenged and therefore entitled to choose the weapons, selecting two pork sausages, a normal sausage and another one, loaded with Trichinella larvae [i.e. roundworm]. His challenger declined the proposition as too risky.
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http://www.spectator.co.uk/books/books-secondary-feature/9210581/the-road-to-bestsellerdom-by-christopher-maclehose-article/
Flashman was launched in Edinburgh, of all places. The Ullsteins came from London; the American publisher, Sidney Kramer, and his wife came from New York. In the course of dinner at Prestonfield House, punctuated by the shrieks of peacocks, it began to dawn on Kramer that it was a novel that he had acquired. Yet he never wavered.