The first one, published in 1953, included the stories "Life-Line," ""Let There Be Light"," "The Roads Must Roll," "Blowups Happen," "The Man Who Sold the Moon," and "Requiem."
The second anthology included only four of those stories, omitting "Life-Line" and "Blowups Happen." Both anthologies included a Heinlein timeline for his stories, characters, and events, and the first had an Introduction by John W. Campbell, Jr