For those who have not read the Rolling Stone article, here is what it has to say:
Kramer has a first name. There is, in fact, an actual Kramer. That Kramer lived in New York, across the hall from Larry David, the writer who created SEINFELD with his friend Seinfeld. "He was a jack-of- all-trades -- he still is," says Richards of his prototype. "He's just a hustler." Kramer, it's said, was the sort of man who paid $400 rent on an $1800 apartment. He lived by mysterious means. He had free access to Larry David's place. His first name, like his fictional counterpart's, is a secret. "We just never talk about it," says Larry Charles, SEINFELD's supervising producer, who gave TV Kramer his unspoken name. "It's a funny name, too," teases Larry David. If Richards knows it, he doesn't say.