"It's the eureka moment where you find that missing piece of the puzzle," said Roberta Olson, curator of drawings at the New York Historical Society.
The society has all 435 original watercolors for Audubon's Birds of America. One is of a similar grouse - the pinnated grouse - and the society dates it to 1824, the same year Audubon supposedly made the image for Fairman.
To Olson, everything fits. Other wildlife artists of the day had found that making drawings for banknotes gave them not only money to live on but a certain cachet. So why not Audubon?
Peck said he thought Audubon's little grouse drawing worked against him. To be sure, it was different from all the eagles, and was not the sort of thing counterfeiters might use or copy. It also showed his prowess as an ornithologist, something he longed for in the stuffy Philadelphia science world that had rejected him.
But the grouse was, in a word, odd.
Bank managers who wanted emblems of security, nationalism, or patriotism might have shied away from it.
"A skittish grouse known for its shy behavior, and running, would not instill in the customer a great sense of confidence," Peck said. "But Audubon was so swept up in his own love of birds and his knowledge of their intimate behavior."
Peck and Newman know they may never learn the whole story. So much is gone.
The engraver went out of business in 1830. The Trenton and Camden banks failed.
The birds - also known as heath hens - have gone extinct. It's yet another detail that resonates among scholars of Audubon, whom many credit with the birth of the conservation movement in this country.
But neither Peck nor Newman can resist the tantalizing possibility that banknotes with Audubon's grouse on them were printed and still exist . . . somewhere.
Given the Trenton and Camden bank connections, they can't help but imagine some tucked away in a Philadelphia attic or stuffed into a Camden wall for insulation.
"That is always possible," Newman said. "Always."
Contact staff writer Sandy Bauers at 215-854-5147 or sbauers@phillynews.com.