Complex sentence: The keys are gold and silver, with the gold key placed to dexter (viewer's left) on the personal arms of the Pope, although two silver keys or two gold keys were used late into the 16th century.
Split into simpler sentences: The keys are gold and silver, with the gold key placed to dexter (viewer's left) on the personal arms of the Pope. Two silver keys or two gold keys were used late into the 16th century.