SoDataSensor(3) on Linux
The text 1fc0c8636569, as Debian 4.0 (Etch) 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.