A determination on a claim by the agency of original jurisdiction of which the claimant is properly notified is final if an appeal is not perfected as prescribed in § 19.52 of this chapter. If no Notice of Disagreement is filed as prescribed in subpart C of this part, the claim shall not thereafter be readjudicated or allowed, except as provided by 38 U.S.C. 5104B or 5108 , or by regulation.
38 C.F.R. §20.1103