After watching some taped episode, I can see:
-In the first season, the bike was a green Klein, but somehow, the 
 fork seemed to be on backward.
-Cannondale Super Killer V appeared in one episode in the second 
 season, but was later rumored to be stolen.  
-The green Klein is back with the fork mounted the right way. 

Charles Puffe [cpuffe@mail1.nai.net]
The bikes in "Seinfeld's" apartment are actually the personal mountain
bikes of actor Michael Richards, "Kramer" on the show.  Jerry does not
ride.