Complex sentence: Originally this was done in wartime to distinguish the copper-zinc coins from pennies, but in later years during this period, this distinctive shape was also retained when the coin was produced in 99.9% nickel or in chrome-plated steel.
Split into simpler sentences: Originally this was done to distinguish the copper-zinc coins from pennies. But the characteristic shape was retained when the coin was later produced in 99.9% nickel or in chrome-plated steel.