Hello from Colorado: It appears to me Airstream heritage is showing some of the more popular models for a particular decade. The Excella 500 is a Sovereign
marketing twist on the 31' trailer. It is the first model with the side moldings going all the way to the front of the trailer.
Here is a link to the Airstream document archives. It is where Airstream has posted all their sales brochures and other documents they used through the years. I find it interesting to research models, specifications, and floor plans.
name was used by Airstream for some 40 years, usually on the longest trailer in the line up.
My friend and I are renovating his 76 Sovereign this last winter. We are getting close to getting it on the road again.