The second argument is the event argument.
It is not stored by the framework.
However you can easily store it in a variable of your own and access it later.
You only need to replace the __doPostBack
with your own function.
var orignalDoPostback = __doPostBack;
var lastEventArgument = "";
__doPostBack = function(eventTarget, eventArgument)
{
lastEventArgument = eventArgument;
orignalDoPostback(eventTarget, eventArgument);
}
Then you can use it as required.