Earth's Last Fortress is a variant title of Recruiting Station (1942).
- The publication date and printing number are not stated
- 1960 is based on Tuck, Reginald1, Clute and Currey and the following:
- The copyright for Earth's Last Fortress is stated as "Copyright ©, 1960, by Ace Books / Magazine version entitled Recruiting Station appeared in Astounding Science Fiction and is copyright, 1942, by Street & Smith Publications, Inc.
- The copyright for Lost in Space is stated as "Copyright ©, 1959 by George O. Smith / Copyright, 1954, by Better Publishing, Inc. / An Ace Books, by arrangement with Thomas Bouregy & Co."
- There are no advertisement or related material that could be used to date the publication.
- The cover for Earth's Last Fortress has the statement "First Book Publication." 114 pages
- The cover for Lost in Space has the statement "Complete & Unabridged." 142 pages
- The cover artists are not credited nor are signatures visible.
- ACE Image Library credits Emshwiller with Earth's Last Fortress and Valigursky with Lost in Space
