According to Google the U6430 is the Windtunnel Ultra, with the onboard tools at the top behind the clear visor and self propelled.
I have the U6425-900 myself, which is a smaller model number, but mine is the base model. Only the self propelled feature, no onboard tool rack and is slightly shorter.
It would take a lot of work, but if someone wants a few hours to kill, I think if you comb newspapers.com (trial required) you might be able to find old newspaper ads for each WindTunnel and then just sort them by whatever date the newspaper that they appear in was published from oldest to newest.