A savings bank not having capital stock may establish a business trust for the benefit of its depositors, with the approval of the director and subject to such rules as the director may adopt. The director may permit this business trust to become a mutual holding company owning all shares of an interim stock savings bank, the sole purpose of which shall be to merge into the mutual savings bank that formed the business trust. The depositors in an unconverted savings bank which has merged with the subsidiary of such a mutual holding company, in the event of a later conversion of this mutual holding company to the stock form, shall retain all their rights to their deposits in the savings bank, and shall also receive, without payment, nontransferable rights to subscribe for the stock of the holding company, and rights to a liquidation account maintained by the holding company in proportion to their deposits in the savings bank, to the same extent that they would receive these rights in a stock conversion of the savings bank as prescribed in chapter 32.32 RCW.