Percussion with a pleximeter is an early example of sound being used as an adjunct to diagnostic medicine and was the precursor to sonar and ultrasound.
The pleximeter was placed on the chest wall and used as a platform for percussion with a hammer. The nature of the resulting sound could help to distinguish healthy lung from fluid or consolidation and was also used to map out the size and shape of underlying organs.
A fine example with a graduated scale on the ivory and hinged wings with ornate silver work. There is a similar piece shown on page 194 of Elisabeth Bennion’s book “Antique Medical instruments” which is is the Museum of historical Medicine in Copenhagen.