If bonds of the issue for which the sinking fund is established are offered at a price not in excess of par and accrued interest or at a price not in excess of the lowest price at which the bonds of any other political subdivision of this State are offered, then the sinking fund shall be invested in such bonds. Otherwise it shall be invested in such other bonds as are offered at the lowest price, due regard being had, however, to the legality of such issue and the solvency of the maker or, in the discretion of the custodian, in bonds of the United States of America or of this State. In the purchase of bonds the custodian shall, if possible, purchase bonds with maturities coincident, or as nearly as possible coincident, with the maturities of the bonds for which the sinking fund was established.