Complex sentence: One gun was manufactured as a prototype Canon de 220mm L Mle1917 Schneider (FAHM) self-propelled gun on a dedicated tracked, unarmoured chassis with the optional armour shield.
Split into simpler sentences: One gun was manufactured as a prototype self-propelled gun. The "Canon de 220mm L Mle1917 Schneider (FAHM)" was the gun mounted on a tracked, but unarmoured, chassis with an optional armour shield.