If any district authorized the issuance of refunding bonds prior to August 14, 1929, and the same or any part thereof are unsold, the board may call an election in the manner and upon the notice prescribed for the issuance of refunding bonds, at which election there shall be submitted the question of whether such refunding bonds of any maturity shall be sold or exchanged to refund any maturity of outstanding bonds of the district as the board shall determine, or as may otherwise be specified in the notice of such election.