An assured may cancel an Open Cargo Policy by delivering to the underwriting agent, at least fifteen (15) days prior to the requested date of cancellation, an application for cancellation executed by the assured on Form MA-304, prescribed in §308.524, together with the original policy. The policy shall be cancelled as of the effective date requested in the application, which, unless otherwise agreed by the Maritime Administrator in writing, shall not be a date earlier than fifteen (15) days following the date of the receipt of the application as acknowledged by the underwriting agent on Form MA-304, with respect to all risks that have not attached prior to said effective date. Such cancellation shall not relieve the assured of the obligation to file closing reports with respect to all risks which attached prior to the effective date of the cancellation and to pay all unpaid premiums. Within four (4) months of the effective date of cancellation, unless otherwise agreed by the Maritime Administrator in writing, the assured must file a closing report in duplicate on Form MA-313, prescribed in §308.533, of all shipments covered by the policy for which closing reports have not been previously filed. The assured shall mark this closing report ”Final Closing Report on Cancellation of Policy”, and file a certificate on Form MA-313-B, prescribed in §308.535, executed by the assured in duplicate. Thereafter, when all unpaid premiums have been paid, the assured will become entitled to a refund of the collateral deposit, or cancellation of the surety bond in accordance with §§308.509 and 308.510. If the assured has lost or mislaid the original policy and is unable to produce it for cancellation, the assured shall execute a letter of indemnity and such other documents as may be required by the Maritime Administrator.