Within a six-month's period from date of issuance of the permit to export firearms, the exporter shall furnish or cause to be furnished to the Director (a) the certificate of exportation (Part 3 of Form 9 (Firearms)) executed by the District Director of Customs as provided in §479.117, or (b) the certificate of mailing by parcel post (Part 4 of Form 9 (Firearms)) executed by the postmaster of the post office receiving the parcel containing the firearm, or (c) a certificate of landing executed by a Customs officer of the foreign country to which the firearm is exported, or (d) a sworn statement of the foreign consignee covering the receipt of the firearm, or (e) the return receipt, or a reproduced copy thereof, signed by the addressee or his agent, where the shipment of a firearm was made by insured or registered parcel post. Issuance of a permit to export a firearm and furnishing of evidence establishing such exportation under this section will relieve the actual exporter and the person selling to the exporter for exportation from transfer tax liability. Where satisfactory evidence of exportation of a firearm is not furnished within the stated period, the transfer tax will be assessed.