I think it was under "The Day After Tomorrow" title when published under the psuedonym of Anson MacDonald...
See post #88.
A bunch of his stories were published under 'Anson MacDonald'.
I still have my paperback copy of 'The Day After Tomorrow' and it has the author as Robert Heinlein. It also says on the cover something to the effect of, "Previously published as 'The Sixth Column'. That's the only way I knew that both books were one and the same.