I'm not really sure what you're expecting - None of the major car rental companies will promise a specific model on standard rental. That's exactly why the "or similar" language is in there. On the convertibles, the Mustang generally is the most popular model in rental car fleets, but the Chrysler 200 or Chevy Camaro are also mixed in there. If you want any better information, you'll have to contact the rental location directly.

If you want a specific model, then most rental car companies - including Avis - do offer specialty car rentals where you can reserve a specific model. However, there is usually a significant extra charge to go that route.