I had no idea, until doing a random Google search to get a puzzle idea, that "back number" is actually a phrase/idiom from the late 1800s...
Henry David Thoreau - Walden
California became a state
Lewis Carroll - Through the Looking Glass
Year Alaska was purchased
Leo Tolstoy - War and Peace
North Dakota became a state
Massacre at Wounded Knee, SD
Washington became a state
Maxim invented the machine gun
Year William Henry Harrison was President
Eastman Kodak began amateur photography
Fredrich Nietzche - Thus Spoke Zarathustra
Nathaniel Hawthorn - The Scarlet Letter
You can check your answers for this puzzle on Geochecker.com.