This is the rubber diaphragm that centers the coil in the instrument. It works for both the TruTone(tm) or SolaTone(tm) speech aids.

Note: this is an advanced repair. Care must be taken not to damage the coil wires when removing the original diaphragm. You must also keep the blue and gray coil wires in their original position. Look at the wires below the diaphragm in the picture for an example.