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.
According to Heinlein there were two reasons for his using psuedonyms. First was that Campbell did not want to have more than one Heinlein story per issue in his magazine... and Heinlein was so prolific that he often had two or three stories published in the same issue... just under different names. Secondly, even the great Heinlein was occassionally got rejection notices from Campbell and those not-up-to-snuff stories got published elsewhere under a psuedonym.