If you have a good eye for dimensions, and are casually walking around a lot and want to quickly know whether it's an E-350 or E-450, you can see the difference at the rear wheels.
The narrower 94-inch body of the Axis/Vegas was likely designed for the narrower track width of the E-350. When Thor switched to the wider E-450 chassis, the rear tires clearly stick out beyond the body. I can spot the difference easily.
Obviously if knowing is important it has to be verified through VIN.
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