You're correct but Nikon called it a Shutter Operation Mode Selector. It does lock the shutter release but that is secondary (according to the manual). Its main function is selecting whether the camera is used with or without the motor drive. Evidently it was possible to damage the camera if it wasn't set to the MD position when using a motor. The trouble was that few owners were aware of that and failed to set it correctly so Nikon eliminated it in an updated version.
Originally Posted by Rol_Lei Nut