An allowance shall be made in the assessment of duties for deficiencies in the contents of packages when, before the liquidation of the entry becomes final, the importer files:
(a) In the case of a concealed shortage, a Customs Form 5931, in triplicate, executed by the importer alone, and the port director satisfies himself as to the validity of the claim; or,
(b) In the case of an unconcealed shortage, a Customs Form 5931, in triplicate, executed by both the importer and the importing or bonded carrier, as appropriate.