31 CFR 306.40: Execution of assignments.

The assignment of a registered security should be executed by the owner, or his or her authorized representative, in the presence of an individual authorized to certify assignments. All assignments must be made on the backs of the securities, unless otherwise authorized by the Bureau. An assignment by mark (X) must be witnessed not only by a certifying individual, but also by at least one other person, who should add an endorsement substantially as follows: “Witness to signature by mark,” followed by the witness' signature and address.