If a claim for drawback of tax is not allowed in full, the appropriate TTB officer shall notify the claimant in writing of the reasons for any disallowance.
27 C.F.R. § 28.335
46 Stat. 690, as amended, 48 Stat. 999, as amended, 72 Stat. 1335, 1336; 19 U.S.C. 1309 , 81c , 26 U.S.C. 5055, 5062