Bonds issued pursuant to this chapter shall be in the form of negotiable coupon bonds, payable to bearer, with the privilege to the holder of having them registered as to principal on the books of the treasurer of the county and the principal thus made payable to the registered holder (unless the last registered transfer shall have been to bearer) upon such conditions as the authorities may prescribe. Unless registered such bonds shall have all qualities of negotiable instruments under the law merchant and the Negotiable Instruments Law. Provided, however, any bonds issued pursuant to this chapter and purchased by the United States or any agency or department thereof, may be in fully registered form as to both principal and interest, and may be registered on the books of the treasurer of the county.