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