ur right...here's what faq.org says about it
"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.
The bikes in "Seinfeld's" apartment are actually the personal mountain
bikes of actor Michael Richards, "Kramer" on the show. Jerry does not ride."