what was in the box that Henry an Bill were taking to the fort?

There is a dead body in the box.

"Henry, squatting over the fire and settling the pot of coffee with a piece of ice, nodded. Nor did he speak till he had taken his seat on the coffin and begun to eat."

"He indicated the third person by a downward thrust of the thumb to the box on which they sat."

"What gets me, Henry, is what a chap like this, that's a lord or something in his own country, and that's never had to bother about grub nor blankets; why he comes a-buttin' round the Godforsaken ends of the earth--that's what I can't exactly see."

"He might have lived to a ripe old age if he'd stayed at home," Henry agreed."