2 different models of Reverb (left or right doesn't really matter, the button would be slightly on the other side but not a problem). On one, the broken part is bigger in diameter and won't fit ...
I believe it is linked to the year of the Reverb (before and after 2012) so if you know what year it is, you should be fine. Might want to ask your LBS or SRAM to be on the safe side.
(watch out if you buy the replacement kit, there are 2 versions depending the version of the Reverb, learnt that the hard way ^^)