Men used the big knocker on the right because it made a much deeper sound thereby signaling that another man should open the door. Women used the softer sounding left knocker to indicate a woman should answer. Rick Steves learned about these gender specific doors knockers when he visited the village of Abyaneh for his PBS documentary (check it out here).
These door knockers can still be found, usually in small, old villages. I came across this particular door while passing through Jowestan, one of the 82 villages that makes up Taleghan.