The image shows a dog whistle called the "Shepherd's Whistle" made by Acme. This whistle is a variable-pitch whistle used to give commands to sheepdogs. The user holds the semi-circular part in their mouth and blows into it to produce a high-pitched sound. It is a type of dog whistle that can produce high-frequency sounds that are inaudible to humans. Acme is a long-established whistle manufacturer in the UK, and this whistle is also made by Acme.