This caught on in a big way, and the period 1910 1922 is known in slot machine circles as the Gum Age . Previously, slot machines tended to use playing card symbols, with the obvious gambling overtones, so Mills changed to fruit symbols, the flavors, and the bar was the gum package.
Early bars were apparently an advert for the gum produced by the successful machine manufacturer, the Bell-Fruit Gum Company. Another evolution in the name of slots was 'fruit machines,' and it seems quite obvious where that came from.
