Apparently this is a myth. A book published in 2005, and reported in The Telegraph, claimed firstly that there is no evidence that the cannons came from Sebastopol, and secondly that they weren't used until 1914. It also says that "the ingot" (of bronze taken from these cannons) wasn't used between 1942 and 1945 because it was mislaid. It does appear to have been found after the end of World War II, and the Telegraph article says that there's enough metal left to make 85 more medals.
The cannons that were first used in 1914 are of Chinese origin, and where they actually came from is a mystery.
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