Introduced in 1937 by the Mills Novelty Company (Reintroduced as the Hi-Top BONUS in 1948)
Before acquiring this machine, I last played a Horsehead Bonus in Yokosuka, Japan in 1955-56. It was located in a small Japanese coffee shop. It used a 10-Yen coin about the size of a quarter. The exchange rate at that time was Y360 to $1 so playing the machine was inexpensive. And it was fun.
The first reel of the machine has the letters B-O-N-U-S superimposed over the regular symbols, 4 symbols apart. This is a 20-stop machine so the letters are evenly distributed.
As the letters B-O-N-U-S appear on the payline, in sequential order, they are displayed on the annunciator. They remain displayed until the letter S is achieved. The machine then awards 18 coins (the same as three bells) and clears the annunciator for the process to begin again.
The idea of the bonus was to keep the player at the machine longer. To that end it was successful; it is hard to walk away from the machine when you only need the last letter or two.
The odds are 1:20 for each letter on the 20-stop reel so, over time, it averages 100 plays to spell B-O-N-U-S. The 18-coin award is an 18% return to the player. The letter S is superimposed over a lemon so it does not conflict with any normal bell machine awards. The 18% payout for the BONUS is in addition to the payout for the fruit symbol awards.
COMPUTATION OF ODDS, MACHINE CYCLE = 8000 POSSIBLE COMBINATIONS
Hit Frequency: 8000 / 1082 = 7.39
Return to player: 6648 / 8000 = 83.1%
A positive expectation machine.
The expectation of a win increases as a function of how many letters of B-O-N-U-S are showing. This machine has a basic payout of 65%. With the letters B-O-N- showing, the payout percentage would increase to 110%. The table
summarizes the expectation per coin played as a function of progress of the letters B-O-N-U-S showing.
The last column shows in general the value of the BONUS relative to the letter showing.
EXPECTATION - LETTERS SHOWING
Letters Showing Expectation Increase in Odds
None -5% 18%
B- 0 23%
B-O- 7% 30%
B-O-N- 22% 45%
B-O-N-U- 67% 90%
This machine is known as a "NEW BONUS" first introduced in August 1939. It is built with conventional 20-stop reels, a simplified mechanism. The BONUS machines introduced in 1937 had shifting reel stop levers carried over from FUTURITY, Mills' famous cheating machine.
The original BONUS was a 10-stop machine utilizing 20-symbol reels. The reels could stop on only 10 symbols of the 20 depending on whether the reel levers were shifted. This makes the machine two 10-stop, 1,000 combination machies in one cabinet. There are 2,000 combinations available.
On the FUTURITY, the levers shifted twice in 10 plays to almost guarantee a Cherry-Cherry-any winner (80%) to avoid paying 10 coins back to the player without a win in 10 plays. As far as I know, the levers shifted on alternate plays on the BONUS. This removed the possibility of scoring a letter on alternate plays.
A description of the original BONUS and the mathematics of the machine is described in detail here: http://www.joiret.com/mills/bonus/fonctionnement