No bank shall make a loan taking its own stock as security therefor or directly or indirectly purchase shares of its own stock, except in pursuance of provisions of law for reducing its capital stock. No bank shall subscribe for or own capital stock in any other bank except in the usual course of business in payment of an indebtedness or in order to prevent a loss on a debt owing to it and the bank must sell said stock within one year from the time the same is acquired unless this time period is extended by the superintendent; provided, however, that nothing in this section shall prohibit the ownership by any bank of any stock in another bank acquired prior to May 28, 1980.