The literature about the history of Punch and Judy and the Punchmen who kept the tradition alive is full of references to the swazzle. It is usually described as a small device which when placed in the puppeteer's mouth and properly used ( not inadvertently swallowed! ) generates the unique sound of Punch's voice.
Richard Landon has put together a very easy to understand set
of diagrams and instructions showing How to Construct the Swazzle (25k). They are presented here with his permission.