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The first to second gen was easy, RX 300 to 330. Each generation used its own engine....
Midway through second gen the 330 became the 350 because of the engine (going from the 3.3 liter V6 to the 3.5 V6). This though did not signify a new generation, but did give the 07-09 second gen's quite an advantage over the 04-06's.
The third gen started with the '10 model year and has quite a different look to it.