This is a wooden letterhead block from Dr. Joseph Chamberlain. The letterhead block has clearly been used. It is wooden with a metal plate on top showing an engraving of the Adler Planetarium.
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This letterhead belonged to Joseph M. Chamberlain, EdD, the director of the Adler Planetarium from from 1968 to 1991. During Dr. Chamberlain's tenure he served as president of the American Association of Museums, updated museum technology, and hired astronomers, curators, educators, and other professional museum administrators. He increased space with underground additions, adding new theaters and exhibit spaces. Dr. Chamberlain also oversaw the building of the Doane Observatory. We can suspect that Dr. Chamberlain utilized this letterhead while in office based on the wear of the letterhead, information from Adler employees during his tenure, and it having been found in the Adler Planetarium's executive office suite.