SoDataSensor(3) on Linux
The text 2407b85535c1, as Debian 6.0 (Squeeze) shipped it.
The SoDataSensor class is the abstract base class for sensors monitoring changes in a scene graph. If you need to know when a particular entity (as a field or a node) changes, subclasses of SoDataSensor can be used to monitor the entity and notify you when it changes.