Mr. Lyons, the foundation’s executive director, called. The man in Illinois said the stamp had been in his family for generations, most of the time in one safe deposit box or another.

To stamp experts like Mr. Lyons, No. 49 was an unusually treasured find. It was in pristine condition. It never had a gummed hinge affixed to the back, for mounting in a stamp album. It was never recut and reperforated, as one Inverted Jenny was after it was stolen. It was never sucked up by a vacuum cleaner, as another was. It was never put in a locket, as still another was, as a present for an owner’s wife.

Most of all, it was never resold, although Mr. Lyons suspects the man is considering selling it.

So for 100 years, No. 49 remained off the radar, to use an expression that did not come along until decades after the Jennies had captured the public’s imagination. A 1986 book that tracked each of the 100 Jennies had this entry for No. 49: “No record.” No. 49 has been a black hole on a Jenny website set up by Siegel Auction Galleries, a Manhattan firm that has sold many Jennies over the years.

Mr. Lyons said the Illinois man’s 91-year-old father had been a stamp collector, but the stamp had come from his mother’s side of the family. A great-uncle apparently bought it after the sheet of 100 was broken up, and after the great-uncle died, the great-aunt left it to the man’s mother in the 1930s.