20 C.F.R. § 219.31 Evidence of a valid ceremonial marriage.
Title 20 - Employees' Benefits
(a) Preferred evidence. Preferred evidence of a ceremonial marriage is— (1) A copy of the public record of the marriage, certified by the custodian of the record or by a Board employee; (2) A copy of a church record of the marriage certified by the custodian of the record or by a Board employee; or (3) The original certificate of marriage. (b) Other evidence of a ceremonial marriage. If preferred evidence of a ceremonial marriage cannot be obtained, the applicant must state the reason therefor in writing and submit either— (1) A sworn statement of the clergyman or official who performed the marriage ceremony; or (2) Other convincing evidence, such as the sworn statements of at least two persons who have direct knowledge of the marriage, preferably eyewitnesses to the marriage ceremony.
Title 20: Employees' Benefits
PART 219—EVIDENCE REQUIRED FOR PAYMENT
Subpart C—Evidence of Relationship
§ 219.31 Evidence of a valid ceremonial marriage.

