Consolidated transportation bonds shall be executed on behalf of the Department by the manual or facsimile signature of the Secretary. Other signatures on the bonds may be either manual or facsimile. If an individual whose manual or facsimile signature appears on any consolidated transportation bond or coupon ceases to serve in an authorized capacity before the delivery of the bond, the signature nevertheless is as valid and sufficient for all purposes as if the individual had remained in that capacity until delivery of the bond.