First of all, what is the dead man's hand? From Wikipedia;
The make up of poker's dead man's hand has varied through the years.
Currently, the dead man's hand is described as a two-pair poker hand
consisting of A♣A♠8♣8♠. Along with an unknown "hole"
card, these were the cards reportedly held by "Old West" folk hero,
lawman and gunfighter, Wild Bill Hickok, when he was assassinated.
In the fift card section on the same page;
In accounts that mention two aces and eights, there are various claims
regarding the identity of Hickok's fifth card, suggestions that he had
discarded one card or that the draw was interrupted by the shooting
and Hickok therefore never received his fifth card.
Historical displays in the town of Deadwood, including one in a
reconstruction of the original saloon, display the 9⋄ as
the fifth card. The Lucky Nugget Gambling Hall, at the historic site
of Nuttal & Mann's Saloon, displays a J⋄ instead. The
Adams Museum in Deadwood has a display that claims to be the actual
Bulldog Squeezers Playing Cards held by Hickok ( A⋄ , A♣, 8♥, 8♠, and the Q♥).
The old Stardust Casino on the Las Vegas Strip had used a 5⋄
in related displays.
As we can see, there are a lot of rumor about this subject. Also there is a web site called DeadMansHand.us;
The earliest known reference to a dead man's poker hand is from 1886
and it is there described as three jacks and a pair of tens. It was,
however, not a reference to Wild Bill Hickok's hand. Although the
evidence is lacking, it is generally "accepted" that Hickok's hand was
A♠, A♣, 8♠ and 8♣. There are many claims as
to what the fifth card was and there is even reason to believe that it
had been discarded and that Bill was drawing his final card when he
was shot dead. Whether the fifth card was queen of diamonds, jack of
diamonds, five of diamonds, nine of diamonds or queen of clubs, all of
which have been suggested in various contexts, it is likely that
Hickok was already holding the winning hand and that the fifth card would not have changed anything.
tl;dr
Unfortunately, we never know what was the fifth card!
