In such contracts there shall be a provision that if the age or sex of the person or persons upon whose life or lives the contract is made, or if any of them has been misstated, the amount payable or benefit accruing under the contract shall be such as the stipulated payment or payments to the insurer would have purchased according to the correct age or sex; and that if the insurer shall make or has made any underpayment or underpayments or any overpayment or overpayments on account of any such misstatement, the amount thereof, with interest at the rate to be specified in the contract but not exceeding six percent per annum, shall, in the case of underpayment, be paid the insured or, in the case of overpayment, may be charged against the current or next succeeding payment or payments to be made by the insurer under the contract.