Sliced wrapped bread first appeared in 1930, and that became the sandwich standard right away. They had the slicing technology before then, but they didn’t have the wrapping technology and the two had to go together.
Before sliced bread, the lunch literature is full of advice on social distinctions and the thickness of bread in sandwiches. You slice it very thick and you leave the crusts on if you’re giving them to workers, but for ladies, it should be extremely, extremely thin. Women’s magazines actually published directions on how to get your bread slices thin enough for a ladies lunch. You butter the cut side of the loaf first, and then slice as close to the butter as you possibly can.