It depends exactly when in 2009/10 the shifters were made and there is no easy way to determine that, not least because any number of these shifters have now had part or all of their internals changed so a simple inspection from the outside won't tell you very much, really.
At various points during 2009/10 Campagnolo changed:
1. The material and shaping of the cable bushing that sits right at the back of the lever to assist in better cable release
2. The exact profiling of the ball-tracks in the clutches that set the index interval of the lever, partially to improve the register in each gear and partially to make the click-stops more positive and less "light-action" (this was in part in response to consumer demand for a more robust, traditionally Campagnolo feel).
3. The body moulds underwent modification to fix a dimension tolerance problem that only showed up under mass-production conditions and led to custom-shimming of some parts of the lever internals in early versions.
4. At the same time as the internal dimensions of the body were changed and better regulated, the cable ports were slightly modified to make it easier to get the outer cables to seat correctly on a wider variety of 'bar shapes.
5. During early 2010 production the spring in lever 2 was changed slightly.
These were mostly "running changes" and followed Campagnolo's policy of continuous upgrade and change during production, rather than waiting for a model year-end.
In general we'd advise that the more recent versions are the best bet. One way to achieve this at a lower cost might be to buy the older levers and change the right hand body complete (EC-RE100) so as to get the most up-to-date internals and body shell - this is additionally advised as the newer internals are designed to work with the newer Chorus through Super Record RDs, which have a stronger return spring than earlier versions to compensate for the increasing use of more complex or internal cable runs and the need to use in-line cable adjusters on many frames, which can add extra friction on the gear inner cable.
Hope that helps!
Velotech Cycling Ltd
Campagnolo main UK Service Centrewww.velotech-cycling.ltd.uk