Hi,
I went to the Original Parts Group website to look at a picture of the front of your guages. It is a 5 guage cluster with the center being a Tach. The other guages at the top moving clockwise as you are looking at it is Fuel (Gasoline), Temp, Oil Pressure, and Battery. So my first observation is that you mentioned an Ammeter but it is not. It is a voltmeter - Good for you as you do NOT want an Ammeter.

You likely are feeding battery (i.e. voltage meter) part of the guage just by the B+ used for all of the guages. The B+ is a common copper buss similar (but in the center of your cluster) to your ground buss which is routed around the right edge of the guage, as you accurately pointed out. The B+ all connects to a Tab that is the left one of the three near your "F" label.

It looks like the center tab near label "F" is actually your Ground tab. If you look closely to that, it appears that a copper line runs up under the "F" guage connecting it to the Ground Buss. You have this one with an arrow on it labeled TACH in your picture.

My best guess.... is that your Tach is the Brown Wire with the black connector on it.
Hope this helps... it would be nice to see this in person to get a better feel for it but Florida is a bit far.
Jerome