Ammonium Alum and Sodium Thiosulphate form Ammonium Thiosulphate - the alum is the hardener the metabisulphite adjusts the pH to the required acidity for the hardening to occur.

That's why the Kodak fixer is a Rapid hardening Fixer.

Alkaline fixers give off ammonia, odourless are the acidic side of neutral, and depend on heavy buffering to prevent them turning alkali.

Ian