It may have been published without a copyright notice? As it was published between 1927 and 1977, that would explain why it's in the public domain: https://comm

It may have been published without a copyright notice? As it was published between 1927 and 1977, that would explain why it's in the public domain: https://commons.m.wikimedia.org/wiki/Commons:Hirtle_chart https://commons.m.wikimedia.org/wiki/Commons:Hirtle_chart