I beg to differ with regards to the "O" standing for Overlander...that's based on a faulty document that has roamed around in the Airstream archives for years. It mistakenly indicates a "0" (zero) as an "O" (letter). It stated the "O" stands for Overlander when in actuality it is intended as a "0" (zero) which is just a placeholder when the "Land Yacht" level of finish is present. "I" was used for "International" finishes.
The term "Overlander" has always stood for a length of trailer, not a level of finish.
Regardless, the "T" indicates a twin bed layout.