Contributions made by a member pursuant to Section 20630, Government Code, in excess of normal contributions, shall be refunded on written application filed in the office of the board at any time; provided, however, that such refund may not be made within one year following the date of any previous refund of additional contributions, and provided further that a partial refund shall not be made. Such refund may be authorized by the board notwithstanding the limitations herein with respect to the date of previous refund upon a showing of good cause therefor.