Kuriositas has the answer.
Letâ€™s take the bell symbol, as a starting point. You might imagine â€“ and itâ€™s a good guess â€“ that as church bells are rung by the winning side at the end of a war, that they are a symbol of victory.
In fact, in the 1800s the first three-reel slot machine game was invented by a Charles Frey and the name he gave it was the Liberty Bell â€“ no guessing what the winning line was. This first design was duplicated by many in the gameâ€™s development and the bell was retained in those new designs and is still with us today.
The bar symbol is a little less obvious. We might think that they represent bars of gold but strangely enough they represent a pack of chewing gum! It was the logo of the Bell-Fruit Gum Company. So why on earth would this symbol appear on a slot machine?
The answer lies in the gambling laws in some US states in the late 1800s where betting was prohibited altogether. Even normal vending machines were regarded with some suspicion by some law-makers. If cash prizes were given then the manufacturers of the machines (not to mention those who rented them for use in their establishments) would be liable to prosecution. So they decided that the prize would beâ€¦ chewing gum!
This not only explains the presence of the bar symbol on slot machines but also the addition of fruit which appeared at the same time. You may have already guessed that the cherry, melon and other fruit that appeared on the reels were indicative of the flavour of gum the winner would receive in recompense for the penny they had placed in the machine.