"a penny slot-machine called 'What the Butler Saw'"
Mutoscope - What the Butler Saw
Creative Commons AttributionMutoscope - What the Butler Saw - Credit: Linda Spashett Storye_book

 What the Butler Saw was a film reel used in a mutoscope - a coin-operated motion-picture device, patented in 1894 and a popular feature of pleasure piers and amusement arcades in the UK.