Pelican P21 & Mentor M21
All the American Pelican Books were eventually reissued as Mentor Books with the prefix letter changed from "P" to "M". Number 21 was the only one that was not an otherwise identical reissue. The Pelican edition was a translation of The Odyssey that was done for British Penguin in the mid 1930's. When NAL split from Penguin they lost the rights to that translation. When Pelican #21 was reissued as a Mentor Book they substituted a different translation.
(Comments and Mentor scan are by Kenneth R. Johnson)