Sold by the Colorscope Corporation of Moline, Illinois in the 1940s and 1950s, the Colorscope stereoscope was a contemporary and direct competitor to the more famous Viewmaster which used rectangular cards rather than rotating reels to hold multiple stereo pairs and feed them through the viewer.

SOURCES

https://vintageviewers.com/colorscope-magicolor-stereoscope

https://www.viewmaster.co.uk/htm/magicolor.asp

https://www.viewmaster.co.uk/htm/conway.asp