SoCallbackAction(3) on Linux
The text 6b473472271e, as Debian 7 (Wheezy) shipped it.
The SoCallbackAction class invokes callbacks at specific nodes. This action has mechanisms for tracking traversal position and traversal state. In combination with the ability to pass geometry primitives to callback actions set by the user, this does for instance make it rather straightforward to extract the geometry of a scene graph.