I'm gonna say that cavity is the black power feed from the alternator on an a-body. Commom problem area. The terminal overheated due to excessive resistance. You might consider an ammeter bypass mod. At the very least read about what's going on or went on to cause this here: http://www.madelectrical.com/electricaltech/amp-gauges.shtml

Evans has this half of the connector and the individual male plugs available.