A module for removable insertion into a body of an instrument having longitudinal strings, the body defining a transverse cavity extending from a lateral edge. The module includes a base configured to engage the cavity as the module travels along an axis of movement into the cavity. A first ball plunger contact is configured to conductively engage a corresponding first contact plate affixed to the body when the module reaches at a first point in movement along the axis and to remain engaged as the module moves into cavity along the axis of movement. A second ball plunger is configured to conductively engage a corresponding second contact plate affixed to the body when the module reaches a second point displaced inwardly from the first point in movement along the axis and to remain engaged as the module continues to move into cavity along the axis of movement.