Nobody really knows but there's tons of speculation.
Check out wikipedia:
http://en.wikipedia.org/wiki/Dead_man%27s_hand
Jack or Queen of Diamonds, 5 of Diamonds, 9 of Diamonds, Queen of Clubs...
Being 5 card draw he may have not even have had a 5th card, some say he was shot after he discarded it, before he was dealt a new one.
The earliest reference to "dead man's hand" actually says it's "three jacks and a pair of tens" back in 1886.
Being an avid poker player, I've been hearing this for a long time now!