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davidcyclebackAs far as the King cabinet goes, you evidently are related to him. That it comes from the family (you being a relative), and has been identified as King by his living granddaughter who knew him, is known as family provenance. A letter from the King's descendant (you) would be considered significant documentation. Quickly comparing to the other pics, it does resemble King. That is was photographed by a St. Louis studio is obviously relevant, as he played for the St. Louis Browns.
A potential buyer would compare pics himself to see if he agreed it looked like King, but a family letter, along with the St. Louis stamp, would be considered important documentation. I don't know what buyer of a photo of Buck Ewing or Eddie Plank or Ernest Hemmingway wouldn't think it was great that it came with a letter from the person's descendant, even if the buyer already firmly believed it was the famous person.
As far as SGC goes, I'm sure they don't consider themselves in the business of doing FBI facial recognition or geneological studies-- in particular due to the time required versus the set fee charged. I don't interpret them not accepting the card as a comment on the identity, but that that type of face recognition of unidentified players is not their job.