Gov. Code, § 29917

(a) The bonds shall be signed by the chairperson of the board of supervisors or by any other member thereof as the board of supervisors shall, by resolution adopted by a four-fifths vote of all its members, authorize and designate for that purpose, and also signed by the treasurer of the county, and shall be countersigned by the clerk thereof. All the signatures and countersignatures may be printed, lithographed, engraved, or otherwise mechanically reproduced except that one of the signatures or countersignatures to the bonds shall be manually affixed. Any signature may be affixed in accordance with the provisions of the Uniform Facsimile Signatures of Public Officials Act, Chapter 6 (commencing with Section 5500) of Division 6 of Title 1 of the Government Code.

(b) Notwithstanding subdivision (a), the board of supervisors may, in its discretion, determine that all of the required signatures and countersignatures shall be by facsimiles, provided however that the bonds shall not be valid or become obligatory for any purpose until manually signed by an authenticating agent duly appointed by the board or its authorized designee.