The jackpot actually has two compartments. The way the machine works is it fills the coin tube first, then as the coin tube overflows it starts to fill a hidden jackpot hopper. When that hidden jackpot overflows you will then start to see the jackpot with the glass window start to fill. Finally when that jackpot fills the overflow goes to the proprietors box as profit.
What should happen when you hit the jackpot the coins in the glass window should drop to the payout tray along with 20 coins from the tube.
On the very next pull the glass jackpot door will close, then on the pull after that the hidden jackpot should dump into the glass window jackpot filling it about half way.
The the whole process starts again.
They did this because no one wants to play a machine with zero coins in the jackpot. In this way it has a head start.
The glass window jackpot will not start to finish filling until the hidden one starts to overflow.
I love the old mechanical machines they were pretty well thought out even back then on how to take your money.
Rich