(decedent's spouse was U.S. resident but citizen of Bolivia; under I.R.C. Sec. 2056, non-U.S. citizenship of surviving spouse bars decedent's estate from receiving marital deduction for property passive to surviving spouse unless U.S. citizenship obtained before estate tax return filed; executor's legal counsel advised late filing of return could be undertaken without incurring penalty to permit surviving spouse to obtain U.S. citizenship; estate tax timely paid in amount due if marital deduction inapplicable, but return filed late (even after receiving six month extension); IRS imposed penalty on late-filed return and estate sought refund; refund disallowed and IRS position upheld; Treas. Reg. Sec. 20.2056A-1(b) allows a marital deduction for resident alien spouse who becomes U.S. citizen before late return is filed, but does not grant relief from late filing penalty; legal counsel's advice was not interpretation of substantive tax law and could not constitute reasonable cause for delay in filing estate tax return beyond time of surviving spouse becoming naturalized citizen; estate tax return actually filed nine months after surviving spouse became naturalized U.S. citizen - maximum late-filing penalty imposed).