They were first introduced at the 1939 World's Fair in New York as a souvenir, but soon they were everywhere. There is a large group of collectors of the reels and viewers alone.
In 1945 Gordon Smith and Karl Kurtz founded Stereocraft to design and build a stereo camera which would produce images to mount in Viewmaster reels. The idea was to allow customers to create their own personal Viewmaster reels. And so the camera was named the Viewmaster Personal. It was marketed by Sawyers Viewmaster.
Sawyers also marketed other cameras, including a 4x4 cm TLR, the Sawyers Mark IV, which was made by Tokyo Kogaku (which also made the Topcon line) and some inexpensive plastic cameras.
Sawyers Viewmaster was founded in Portland, OR, and I was privileged to be given a tour of their plant. They are currently part of Tyco Toys, and are still marketing Viewmaster reels.