The unproduced Colorforms set, from 1978.

The Amazing Mumford and his Magic Colorforms was an unproduced Sesame Street Colorforms set designed in 1978 by Sesame storybook illustrator Michael J. Smollin. The cardboard playset had two see-through plastic screens, colored red and green. When the vinyl Colorforms piece passed behind one screen, it would show an object. When it moved to the other screen, the piece would show the letter that the object begins with.

This set was not produced, but the basic concept was used in 1988 for two Sesame Colorforms magic sets -- "The Sesame Street First Magic Show" and "The Sesame Street Magic Alphabet Set."