If more than one series of negotiable certificates of indebtedness is issued hereunder, priority of lien and privileges on the revenues of the utility shall depend upon the time of delivery of the certificates, each series enjoying a lien prior and superior to that enjoyed by any other series of certificates subsequently delivered.  However, as to any issue or series of certificates which is authorized as a unit to be delivered from time to time in blocks, it may be provided in the proceedings authorizing the issuance of the certificates that all such series or issues shall be equal as to lien and privileges regardless of the time of delivery.