|Heinlein's The Moon Is a Harsh Mistress:|
serialized Dec. '65 through Mar. '66
Here's an excerpt from the magazine's Wikipedia entry:
[If] achieved its greatest success under editor Frederik Pohl, winning the Hugo Award for best professional magazine three years running from 1966 to 1968. If published many award-winning stories over its 22 years, including Robert A. Heinlein's novel The Moon Is a Harsh Mistress, and Harlan Ellison's short story "I Have No Mouth and I Must Scream." Several well-known writers sold their first story to If; the most successful was Larry Niven, whose story "The Coldest Place" appeared in the December 1964 issue.
If (no pun intended) you have a desire to read some Golden Age science fiction -- online and for free -- then the If archive is awaiting your reading pleasure.