You have looked at the Paintref.com site?... it is listed there as PPG colour 60517....

Also there is a history pdf of an ebay auction for an Autumn Bronze car where owner says had to get PPG to talk to GM Historical to come up with a mix (17 pigments)... (search google for "Pontiac Firebird rare Autumn Bronze pdf", the April 10 2010 link has the info)... so PPG must have it.