A long-overlooked photograph, displayed for decades in the museum of the Stephenson County Historical Society in Freeport, turns out to be a rare historical treasure - the only known original photo of Abraham Lincoln's stepmother.

The approximately 2-by 3-inch photo of Sarah Bush Lincoln was correctly labeled in the museum's permanent Lincoln exhibit. But the county historical society was unaware that only one other photo of Lincoln's stepmother is known to exist, and it is found only in copies - the original is missing.

"We knew it was her. We just didn't know it was such a big deal," said Suzy Beggin, director of the museum.

The photo is an ambrotype, a 19th-century type of photograph made on a glass plate, in a leatherette-covered wooden case. It was donated to the historical society in 1968 and may have been on display since then.

Late last year, an amateur historian visiting the museum to research some Civil War artifacts recognized the resemblance to the other photo of Lincoln's stepmother, which is a slightly different pose.

After getting permission to take it out of its case, the historian found an inscription on the back of the case that read "Sally Bush - Abraham Lincoln's Stepmother - Thomas Lincoln's second wife."

Beggin said she was originally skeptical about the discovery, but the use of "Sally," a common 19th century nickname for Sarah, convinced her.

"That 'Sally' really struck me," she said. "I know quite a bit about Victorian women, and I know that your own husband didn't call you by your first name in public. ... She would have been addressed (in public) as 'Mrs. Lincoln.'"

The use of a familiar nickname meant that "our photo must have initially been owned by one of her relatives or close friends," she said.

Lincoln scholar Wayne Temple, who was asked to authenticate the photo, agreed that the use of the nickname helps indicate the photo is genuine.

"Since only close family members would have called the subject 'Sally,' there can be but little doubt that it is a genuine picture of Lincoln's beloved stepmother," he said.

The fact that the picture has been in the historical society's collection for so long also lends credibility to the claim that the photo is authentic.

While nowadays anything remotely connected to Lincoln is considered highly collectable, in 1968 a picture of his stepmother would have been of little value to most collectors, and therefore not worth faking. And if someone had wanted to falsify a picture of Sarah Lincoln back then, they would not have called her "Sally."

Temple said the photo "is a very important piece of Lincolniana."

Sarah Bush Johnston Lincoln was born Dec. 13, 1788. Her first husband died, and on Dec. 2 1819, she married Thomas Lincoln, whose own first wife had died more than a year earlier. Young Abe would have been 10 years old at the time of the marriage.

Sarah died on April 12, 1869. The photo was probably taken a few years before her death.

The photo was donated to the museum by a Lincoln collector from Freeport, who apparently purchased it specifically to donate to the museum. Records identify it as coming from the "Frank E. Winston Collection, Chicago," but Beggin said she has not been able to identify Winston or trace the photo's history before 1968.

The picture still is displayed in the museum as it was before its importance was discovered, Beggin said.

"We have not changed the display," she said. ""It's in a good area, under glass, locked and well protected from light, heat and humidity."

Beggin said the museum does plan a special event in July, including a reception to honor Sarah.

Since the existence of the photo at the museum was announced in the winter 2003, issue of "The Rail Splitter," a journal for Lincoln collectors, it has generated considerable interest, Beggin said.

"We've had a big increase in visitors coming to the museum," she said. "That's always good."

Doug Pokorski can be reached at 788-1539 or doug.pokorski@sj-r.com.