Certainly nobody will every know for sure but here is a list I found that gives various cards and how they came to be known as the fifth card:

Jack of Diamonds - according to the transcripts of McCall's trial, a witness claims that this was the card.

Five of Diamonds - the town of Deadwood purportedly has this card on display, claiming it to be the actual card.

Nine of Diamonds - according to supposed eyewitness accounts at the time.

Queen of Clubs - Ripley's Believe It Or Not has this card on display.