Another possibility is a fish knife. Fish knves and forks for formal eating of fish had a similar blade shape, and there was a larger version used to cut and serve fish. Given that the blade on the knife in question is sharp, then I think a fish serving knife may be the most likely use for it. Butter knives are notoriously blunt because butter at room temperature is very soft.
Here is a set of a fish knife and fork for serving.
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