SoTextureUnit(3) on Linux
The text f24286fed297, as Debian 6.0 (Squeeze) shipped it.
The SoTextureUnit class is a node for setting the active texture unit. When an SoTextureUnit node is inserted into the scene graph, all subsequent texture nodes (SoTexture2, SoTextureCoordinate2, SoTextureCoordinate3, SoTexture2Transform, SoTexture3Transform, SoTextureCoordinateEnvironment, SoTextureCoordinatePlane and SoComplexity) will affect the texture unit set in the unit field.