I think that is probably the barrel outside diameter from the 2.1 connector data sheet I just pulled up.
http://www.mouser.com/ds/2/393/760-765_cd-371293.pdf
The inside diameter is the 2.1mm that they are sold by usually. As I said from the size relative to the 2.1 on the Uno I think you are looking for a 1.3 mm, I use them instead of 2.1mm for my 5V wall warts so I can't plug a 9V 2.1mm arduino wall wart where a 5V wall wart wants to go. If you have a local electronics supplier you should be able to get a 1.3 mm jack for a couple of bucks and try it and see if it fits. Once you have that connection, without powering it you can try the various pins on the usb connectorwith an ohmmeter to see if you get a connection which should identify ground.
Peter Van Epp