Yet the camera on in the photo is a Pentax. It looks like one of their 6x7 SLRs.The pinterest account hosting that image did not identify the lens, sadly. I did a little googling but couldn't find an exact match. He/she did appear to be a Nikon fan. I found Nikon apparently made a set of two-piece super long telephotos (I saw one at 1200mm). There was a focus mechanism that was separate, and you attached this to one of the lenses (I think there were three of them). But there were other brands of really long lenses, too, so I cannot be sure it was Nikon.
The hex key is for aligning the lens body to the camera body. Now that you have threaded the lens to the adapter and mounted it to the camera, loosen the 3 set screws and turn the lens until the graduations are aligned on top.