All series D2 switches require that rail extension assemblies be attached to the interior side rails in order to mount the switch. These rail extension assemblies are packaged with every series D2 switch. If a rail extension is lost or missing, contact the nearest schneider Electric™ sales office to order a replacement. Plug-on extension assemblies must also be ordered when installing 30 to 200 a plug-on units in blank spaces of a QW interior in the 38 in wide box. These plug-on extension assemblies (which extend the bus) are not the same as the rail extension assemblies packaged with every series D2 switch (which extend only the mounting rails).