Looking at the electrical circuit on Widget's machine it looks to me as though only the bulb is lit on a win. The box, top right, holds a battery and the switch is below the box and the circuit is made by the payout arm moving to the right.
Attached is a simple circuit designed to give a single shock. The switch could be either a low torque micro switch located in the coin slide to give a shock every time the machine is played or the switch could be like that on Widget's machine to give a shock only on a win. I guess on the some of the cheaper original machines the coin slide consisted of two tracks insulated from each other so the penny itself completed the circuit. The circuit need only be closed for a short time to charge the capacitor and the capacitor will hold the charge for a few minutes or until discharged through the player ----- Ouch.
Beware Electricity is Dangerous
and this circuit can be arranged give substantial shock that may be painful / injurious to some persons. I can remember going to Paul Haskel's where he had his machines displayed in his garage. Paul had a selection of shockers in his collection including the well known Pig. Paul persuaded me, against my better judgement, to have a go on a Pickwick with a shocker. I had expected a small jolt but, by Christ, the belt nearly threw me across the garage and my arms felt like they had dislocated from their sockets.
Hope this helps. John.