(a) Claims for permissive and additional service credit under this part shall be corroborated by a statement from the superintendent of schools or custodian of records of the employer for which the service was performed.
(b) Claims for creditable service under this part performed outside the United States or in federal schools within the United States shall be corroborated by a statement from the custodian of records.
(c) When the official records of the service have been destroyed, the claim may be corroborated by one or more affidavits of knowledge of the service, preferably by persons who served with the member at the time the service was performed.