Roberts donated the painting to the Rosenbach a year ago, seeming to resolve the mystery happily.
So Guston was astonished all over again when she received a phone call over the summer from Jeanette Thomas in Sacramento, Calif.
Thomas' husband, John, is Benjamin Gratz's great-great-grandson. The Thomases had seen news of the Rosenbach discovery and acquisition. They were puzzled.
"I thought, 'What's going on here?' " said Jeanette Thomas, who promptly contacted Guston.
"She called and said, 'I thought I had that [portrait], in my living room,' " Guston said the other day.
Guston managed somehow to stay in her chair.
"Tell me about it," she said.
Thomas described the portrait, which hung on a library wall and was signed "TS" and dated 1831 - the same way the pendant portrait of Benjamin Gratz is signed and dated.
The Atlanta Maria - donated by Roberts, Benjamin's great-great-great granddaughter - is not signed, but Guston noted that would not be unusual because Sully had already signed the linked portrait of Benjamin.
But after hearing about the Thomas painting and looking at photographs, Guston hopped on a plane and headed to California.
"I knew she was excited," Thomas said. "She wanted to come right out."
At the Thomas home, Guston said, she saw a portrait that "looked amazing."
After further study, including close examination by Sully expert Carol Soltis, it became apparent the Thomases' Maria was indeed the original work by Sully, painted from life in 1831.
What to make of the Atlanta Maria? It was also clear from examination Sully painted that portrait as well. Where did it come from? The pose and every detail appeared the same in the two pictures.
More study ensued, and the Rosenbach concluded the Atlanta Maria was a copy Sully made of his original painting, probably at the request of other Gratz family members who wanted a remembrance of Maria after her death at age 43 in 1841.
Guston says both Marias are beautiful. But it is clear the California portrait came first.
"You can see the signed one is done from a live subject and that the other is more ethereal," Guston said. "They are both beautiful Sullys.
The Atlanta painting, she added, is unusual because "here we have a Sully copy of a Sully original."
The Thomases decided their portrait belonged at the Rosenbach and they have donated it to the museum, where it joins an extraordinary collection of Gratz materials and paintings of family members.
"We wanted people to see it," Thomas said.
Probably the best-known Gratz is of Maria's sister-in-law, Rebecca, who helped found the Female Association for the Relief of Women and Children in Reduced Circumstances after the Revolutionary War. She was a cofounder of the Philadelphia Orphan Asylum, established the city's first Hebrew school, founded the Female Hebrew Benevolent Society, and is said to have been the model for the heroine in Walter Scott's Ivanhoe.
Benjamin Gratz fought in the War of 1812, studied law at the University of Pennsylvania, and moved with Maria to Lexington, Ky. The portraits were painted during a long return visit to Philadelphia.
The two Maria portraits plus the Sully portrait of Benjamin are now exhibited together at the Rosenbach so visitors can compare them.
In the same room, there is also a Sully portrait of Rebecca, another Sully of Michael Gratz, her father; a Gilbert Stuart portrait of Rachel Gratz Moses, her sister; a G.P.A. Healy portrait of Rachel's husband, Solomon Moses; a Sully of brother and sister Benjamin and Rebecca; and a Stuart portrait of Rebecca's brother Joseph.
The Rosenbach has a wide range of Gratz materials and artifacts, including Rachel's desk, Benjamin's washstand, Joseph's grooming box, and many family books.
Museum founders A.S.W. Rosenbach, famous book dealer and collector, and his younger brother and business partner, Philip, linked their ancestry to the Gratz family in the 18th century, and throughout their lives collected material related to them.
Guston acknowledged, after this latest Gratz discovery, that there could be more. For instance, there is a missing portrait miniature of Maria painted by John Henry Brown in 1844.
Thomas agrees more may be lurking all over the country.
"I wouldn't be surprised if there's another Benjamin out there," she said.
Contact Stephan Salisbury at 215-854-5594, firstname.lastname@example.org, or follow @SPSalisbury on Twitter.