a ‘salmon belly’

“He unrolls the bills, looking at them with a mysterious smile on his fat face. You don’t understand his smile and wonder if he is thinking how he cheated somebody in a poker game; he looks like a gambler.
    The roll interests you, the outside one is what you call a ‘salmon belly.’ It is a yellowback—a big bill.”

—Jack Black, You Can’t Win, 1926. The money is Canadian.