This View-Master Junior (Sawyer's) can easily be dated due to being made from Bakelite (an early 20th plastic). The first View-Master was made in the 1930s by Wlliam Gruber who was fascinated by 19th century stereoscopes (devices which created the illusion of 3-D using two side-by side photographs). He partnered with the Sawyer's company to produce these viewers (which were debuted at the 1939 World's Fair).

The View-Master was marketed through Ed Mayer's photo-finishing, postcard, and greeting card company, Sawyer's Service, Inc. known eventually as Sawyer's, Inc. The partnership led to the retail sales of View-Master viewers and disks. The patent on the viewing device was issued in 1940, on what came to be called the Model A viewer. Within a very short time, the Viewmaster quickly took over the postcard business at Sawyer's.(source: wikipedia)

For more information on View-Master's and reels see: