Above is a fine selection of both commercially produced, and amateur made stereo slides.
Most are Realist Format (1-5/8in. x 4in.) in size; either cardboard, plastic, metal, or glass mounted transparencies, made from the late 1940s through today's contemporary slides. Most are original slides taken by the owners. Commercial Stereo Slides (Realist or similar formats)
Private Amateur Stereo Slides (Realist or similar formats)