It's the first illuminated viewer! Made of Bakelite and manufactured between 1955 and 1972, this viewer is heavy duty, but also prone to cracking and breaking apart (which I learned when I dropped it while taking these photos.
Order yours for over $200 here.
Notice: Over two years later and a friendly buccaroo in the comments correctly identified this as a Model F, not a Model D. Adjust your life plans accordingly.
And now for our link: Make your own custom ViewMaster Picture Disc Reels here!