A mill owner kept six keys to his office safe.

When money disappeared, every keyholder was questioned.

Cryer counted six.

Then he noticed a seventh scratch around the lock.

A duplicate key had been made.

The culprit was the owner's eldest son, who had taken the key impression from wax used to seal a letter.

The duplicate opened the safe perfectly.

Inside Cryer found not only the stolen money, but records of earlier thefts.

The seventh key solved them all.