How lovely to be talking about the nuances of this story again! The numbers that Annie Proulx selects to flesh out her story are so specific that you have to wonder. In this story, she seems to show a preference for even numbers. The first number mentioned is twenty-four, the number of dollars Mr. and Mrs. del Mar left to Ennis and his siblings when they died. And then, in the next column, the number is reversed: forty-two, the number of sheep killed in one lightning strike!
Proulx seems to want to avoid using odd numbers. Instead of saying that Jack and Ennis were 19, she wrote, "Neither of them was 20." If she does use an odd number, she'll pair it somehow as in referring to the rifle as a ".30-.30." She even pairs up the even numbers, as when Aguirre refers to the men as a "pair of deuces going nowhere." I look upon this as a literary reinforcement of the two mens' bond, the yin/yang nature of life (casting a glance at the yin/yang coaster you gave me, friend!) and the life-giving energy of complementary opposites.