After Gold Medal published Matheson's follow-up novel The Shrinking Man in 1956, they brought I Am Legend back into print. The cover trumpeted the fact that this was the 'Second Big Printing!'
Internally, there were a few other modifications from the first printing. Cover artist Stan Meltzoff's name was dropped from the title page, and the last page lost the notation about this never having been a book before.
Several years back I picked up a pristine copy of the book with a label on the inside front cover designating it as a file copy from the printer. Whether it truly was or not, it's an amazingly sharp copy for a 40+ year old paperback.