In the episode where Jerry and Elaine are flying back from Florida, they both get on an airplane that appears to be a 767. (A very large plane) It appears to be a 767 because of the layout of the seats. Before the airplane takes off the camera shows their plane from the outside as being a South West Airlines 737. I am sure it was a 737 because South West only flies this type of airplane. Additionally South West Airlines is widely known for not having different classes. But in this episode a lot of the plot dealt with the fact that Jerry got the opportunity to fly first class while Elaine had to fly coach.