“Without this,” he said, “we very likely wouldn’t be sitting here talking about Alexander Hamilton.”

The wild success of the musical has turned the historical A.Ham himself into a cultural celebrity, and sent museums and other institutions into their back rooms, looking for Hamiltonia to show off. The hundreds of documents at Sotheby’s, which were consigned by a sixth-generation descendant of Alexander and Elizabeth Hamilton, had been stored in a trunk in the family basement, though some items, like that 1777 commission, had been put on a pedestal of sorts over the years.

“I probably shouldn’t say this, but the commission was on my grandfather’s bathroom wall for years,” David Key, a son of the consignor, said in a telephone interview. “He loved seeing it over the lavatory.”

Mr. Key, an agricultural economist who lives in New York City, said that his mother had always taught her children to take pride in their illustrious forebear, but not too much.