Section 177C. No contract or agreement of the type referred to in section one hundred and seventy-seven A with an insurance adviser shall be enforceable by him unless it is in writing, and executed personally in duplicate by the person to be charged or by his legal representative, nor unless one of said duplicates is delivered to, or retained by, such person when it is signed by him, nor unless it plainly specifies the amount of the fee paid or payable by such person and the services to be rendered by such insurance adviser, nor unless it is in a form currently approved by the commissioner.