According to our research, these Nikon SLR's were originally made specifically for underwater photography by National Geographic. But before you buy one and jump into a lagoon with it, do know that the camera itself is not waterproof.
The huge viewfinder was necessary to accomodate the effective, but unweildy underwater housings of 1977.
The auction listing claims that only 50 were made, but some reports claim it was more like 100. According to multiple camera/Nikon-nerd sites, it appears that 25 of the 100 were made and delivered to National Geographic, another 25 were dolled out to respected underwater shooters and the last 50 were "quietly" sold off.