a Man-with-a-barrel/ tap the barrel

Would you tell me whether I am right with my interpretation of the expressions in bold in the following sentence?

Mr. Flower was a Man-with-a-barrel and one who knows how to tap it.

a Man-with-a-barrel = a person having access to the state treasure-house

tap the barrel = dip in the gravy

Thanks for your efforts

Nothing in the context you give us is there any hint of a state treasure house.