20 CFR 219.63: What evidence is required to establish “good cause”.

The Board will ask for the following evidence of “good cause”:

(a) The claimant's signed statement explaining why he or she did not file the application for lump-sum death payment or annuity unpaid at death or the parent's proof of support within the specified two-year period.

(b) If the statement in paragraph (a) of this section or other evidence raises a reasonable doubt as to whether there was good cause, other convincing evidence to establish “good cause”.