This looks like a deeply struck 2004-D nickel. It does have sharp details and nice relief, which probably indicates an earlier die state. Later die state examples of that year can be fuzzy with deterioration or struck shallowly.
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It was minted in Denver - Denver NEVER made proof coins.
Not to split hairs on this particular point, but there are well known examples of proof reverses getting used on circulation strike coins. Dimes come to mind. It would be rare, but not entirely out of the question. Particularly for 2004 and 2005, since all sorts of oddities could have happened around the half-year design changes. We do, for example, have two doubled dies for those years, even though it's a single squeeze hubbing process.