My two dads

Most people know the actor who played Morty Seinfeld was swapped out in Season 1, but Frank Costanza also began as a different actor:

He was replaced by Jerry Stiller for this episode later in syndication.

The two Mortys:

Nod to Jerry's real life father

Jerry's father, Kal, made signs for a living. A quick Easter egg in this episode:

George's real name

Is not Jason Alexander, it's...

JAY GREENSPAN

The actor that portrays Kramer

In the episode The Pilot, the actor Larry Hankin actually originally auditioned for the real part of Kramer, and was the second choice to Michael Richards.