The custodian of any sinking fund required to be established by the laws of this State for the retirement at maturity of bonds, notes or other obligations issued by any political subdivision of the State not maturing within one year from the date of issue shall invest, within six months from the collection of the tax levied for the establishment of such sinking fund, the funds so collected in bonds of the issue for which the sinking fund is created, in other outstanding issues of such political subdivision or in valid outstanding bonds of some other political subdivision of this State or in bonds of this State or the United States of America, with the privilege of changing the investments from such bonds to other such bonds of any such class, if in the opinion of the custodian of such sinking fund such change is necessary or advisable.