Possibly flour, I guess. I have a recipe for potato candy right here in my desk drawer from my stepsister's family. It has butter in it, so I imagine it tastes better than what that guy made. Plus, you add powered sugar until the dough becomes difficult to stir, so it's not an exact recipe.
Potatoes and peanuts were easier to get in the US, but idk about powdered sugar, maybe it was less expensive than granulated sugar. The recipe predates the Great Depression, though. It's at least turn of the century.