Model number is on the lever body, under the hood. Lift the bottom edge of the hood up and you will see it.
6500 = 9 speed and 6600 = 10 speed, but cable travel is the same so they are interchangeable. The number of workable rear cogs is determined by the shifter, *NOT* the derailleur. So, if you use a 6500 shifter you will get a 9 speed drivetrain whichever derailleur you use, with a 6600 shifter it will be 10 speed.
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