The Drive Socket is the black mushroom that sits on top of the blender. There are 2 models. Based on your OmniBlend's serial number, you can determine which drive socket you need.
WHICH DRIVE SOCKET DO YOU NEED?
If your blender's serial number starts with a 4, you need a drive socket 4.
If your blender's serial number starts with a 5, you need a drive socket 5.
You can find your serial number on the bottom of your blender.