Airsoft Sports got available at their store the NPO-AEG VSS-M RIS AEG and here is their take on this AEG... "The airsoft copy, like the real world military weapon, differs from the standard product by the absence of a rail for mounting optics on the receiver; instead, there is a picatinny rail on the receiver cover.

To install bipods or other attachments, a special clamp with three picatinny rails is installed on the PBS.