The article makes it sound like PSA assessed the prints themselves as from the same period as the autographs (1911). PSA is pretty good about getting print age correct.
Heritage uses the term "ironclad provenance" to describe the Mathewson signature. Since they came from the same scrapbook, the Jackson must have the same ironclad provenance:
"Bowen's husband, Bill, first saw the scrapbook about 10 years ago. It was stored in a barn near Cleveland and belonged to a couple whose family was friends with Frank W. Smith, a photographer with the Plain Dealer newspaper."
I think it's really neat how this stuff gets found in barns.
