The first coin-operated gambling devices were invented on the East Coast in the 1880s and were actually more akin to modern video poker machines than slots. They employed a deck of cards distributed across five mechanical revolving drums, with players rewarded for hitting various poker hands. The machines were located in cigar shops and saloons and the prizes consisted of drinks and smokes.

In those pre-regulated days, the reels frequently didn’t contain a full 52-card deck, thus the configuration of the cards across the five drums meant that certain high-paying hands were literally impossible to hit. Nevertheless, thanks to the machines’ popularity, they spread to the West Coast, where they became particularly prevalent in San Francisco. In the City by the Bay in his workshop, a German immigrant and inventor named Charles Fey was credited with inventing the reel slot machine in 1899.

Fey named his machine the "Liberty Bell" and it included a nickel coin acceptor, a payout schedule, and a large handle on the right-hand side for cranking the three wheels, which featured pictures of diamonds, hearts, spades, horseshoes, and cracked Liberty Bells. Lining up three bells in a row generated the highest payout: 10 nickels. Fey’s invention was such an overnight success that he quit his day job to develop more of these "nickel-in-the-slot" machines, which eventually became known simply as slot machines or slots.

The success of Fey’s machines and the others that followed quickly in their path proved to be their initial undoing, however, as a tide of social reform swept the nation and brought drinking and gambling under attack. In early 1909, San Francisco outlawed slots and other cities and states followed suit, giving rise to dramatic scenes of city dignitaries smashing the machines with sledgehammers or hurling them into the ocean.

But while alcohol would soon fall victim to outright prohibition, the slot machine survived by adding a vending capability to dispense gum, mints, or chocolate with each play. By becoming "venders," the machines circumvented the anti-gaming laws and could be operated anywhere. (Attempts were made to outlaw even these innocuous candy dispensers, but the cases were generally thrown out by the courts.)

And so we come, finally, to the answer to your question. The lemon, cherry, watermelon, orange, and plum symbols corresponded to the flavors of gum. The symbol of the stick, or perhaps the entire package, of gum was first introduced by the Bell-Fruit Gum Company for the highest award or jackpot on its machines. Other manufacturers, such as Pace and Bally, incorporated their logos into the simple rectangular symbol.

When slots were allowed as outright gambling devices once more, the stick-of-gum icon -- and its distinctive long rectangular shape -- remained as the acknowledged symbol for jackpots. By then, the chewing-gum logos had been rendered irrelevant and no longer appeared on the rectangle, which is when the icon became known simply as a bar, since that’s just what it looked like.