OK, not sure exactly how to use this item. Can't find any information anywhere. From what I can tell, it has a bolt pattern of a Nikon SLR, and is threaded for a Leica M39 screw mount lens. Leads me to believe you would replace the lens mount on the Nikon body with this adapter and use Leica M39 lenses. Since it has a thin lip on the mounting part of the adapter I don't think you can mount it over the existing lens mount. I don't have a Nikon body to try it on so I'm giving you all the information I can.

Rare item for sure.