Each bond or other evidence of indebtedness shall contain on its face a statement specifying the purpose for which it is issued and it shall be unlawful for any officer of the city to use the proceeds thereof for any other purpose. Any officer who shall violate this provision shall be deemed guilty of a violation of this charter, except that, whenever the proceeds of any bond issue or parts thereof shall remain unexpended and unencumbered for the purpose for which said bond issue was made, the council may authorize the use of said funds for the retirement of bonds of such issue or for any other purpose permitted by law. All bonds or other evidences of indebtedness issued by the city shall be signed by the mayor and countersigned by the clerk, under the seal of the city. The signatures of the mayor and the clerk, and the seal of the city may be facsimiles in the case of fully-registered bonds. Interest coupons may be executed with the facsimile signature of the mayor and the clerk. A complete and detailed record of all bonds and other evidences of indebtedness issued by the city shall be kept by the clerk or other designated officer. Upon the payment of any bond or other evidence of indebtedness, the same shall be cancelled.