20 CFR § 210.7 - Verification of service claimed.
Service claimed by an employee, which is not credited in the records of the Board, must be verified to the satisfaction of the Board before it may be credited. Verification of the Service claimed shall be as follows:
(c) If the payroll, personnel and detailed records are incomplete or missing, the service claimed and not established by these records will be verified from any other books and records of the employer.
(e) When service is verified as to over-all dates, but is not supported in detail by employer records, and when there are no employer records showing in detail absences from service, a deduction shall be made to cover an average amount of the absences. The deduction shall be the absences shown by the applicant or 5 percent of the total period in question, whichever is greater. However, where the employee submits detailed records of the service claimed, properly identified and established as having been made at the time the employee performed the service for which detailed records of the employer are not available, full credit may be allowed for the service as may be verified from the records. Also, the employee may be permitted to establish in any other manner satisfactory to the Board the actual amount of his or her absences.
(g) For the purpose of verifying service after 1936, employers shall preserve in accessible form the original records of service and compensation for a period of five calendar years after the due date of the report.