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The reason that "gallon" in "five gallon bucket" is not pluralized is because it's not a noun; it's an adjective.

Oh, I totally agree. I think you misunderstood my statement;

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eg When descibing the maximum amount of water that a "Five Gallon Bucket" would hold, you'd say "Five Gallon", "40 Pint" or "22.73 litre". Each gallon, pint or litre is abstract.

Q. "How much water can a five gallon bucket hold?"

A1. "Five gallon."
A2. "40 pint."
A3. "22.73 litre."

The usage of "five gallon" vs "five gallons" in the question was never in doubt. In the question, "five gallon" is, as you've stated, an adjective. In the answer, however, it is not.
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