How They Work - Double or Nothing - Double Win Relay #1
The box the red line is passing through is the probability/random unit disc. The wipers on that unit are already stopped, so as soon as you push the double button and the double play relay powers, the double win #1 relay will power if the wipers are in a position to pass the current through.
The reflex relay branch path is doing the usual reflex function. When you win credits, this path will eventually be disconnected. It reconnects as coins/credits are played.
The yellow circuit path keeps double win relay #1 powered for the remainder of the win detection period
Likelihood of Winning
The outcome of a double attempt depends only on the position of the probability/random unit wipers. No player skill is involved like the double/nothing game on Big Wheel/Magic Ring. As the first game with the feature, Stock Market's probability unit was designed to allow fiddling with the odds.
The main red circuit path on the unit needed to connect wires #71-1 and #80-1. That occured through the rivets marked X. Additional rivets jumpered in are marked in green and labelled IX and XI. If they are all connected per the diagram, you have a 24/50 chance of winning a double through the red circuit.
When the reflex function allowed the reflex relay to close, and additional 4/50 chance is supplied. So when nobody has won significantly for a while, and lots of coins/credits have been played, it is a statistically good bet to try for double. After a big win, the reflex controlled circuit is probably open, and it's a slightly bad bet to go for it.
Of course, an operator who figured out how this stuff works could chop jumpers on the unit and disconnect the extra green rivets..or worse, snip out some of the red X rivets.
