This is a 6 volt system. The wire colors have all faded to white. In the diagram I have circled the post where the wire originates but it was attached to anything. It is just dangling under the dash. Loose ground wire?
http://i1232.photobucket.com/albums/...ns/Capture.png

 

May have found the answer to the question in a search. I think the original wire should have been black with green chasers and run back to the ignition switch.

 

The item you have circled is a metal bar used to supply a common power to all three gauges. And you are correct, that power comes from the GA post on the ignition switch.