Seeing Previous Definitions for "Vagina" One was Ham Wallet. So Man Wallet is that Of a Man or Woman who are Uncertain of What Lies below. Not being Able to See your "Down there" you must Assume you are the Proud owner of a Man Wallet. Sort of an Either Or.
"My oh My Cad. What a Nice Man Wallet you Have"
"Cad uses it's Man Wallet to Lure in Unsuspecting Girls Or Guys"