The serial number for a particular worklist item is the x_y where x is the Process Instance ID for your process instance and y is the Activity Instance Destination ID for your activity. For example, 123_78 would be the serial number for activity instance 78 in process instance 123. Both of these fields are available in the Workflow Context browser (see attached).
Do keep in mind that a serial # is only valid while the activity has not been completed. Once an activity has been completed, it cannot be accessed again.