The Deputy Commissioner may accept for recording in his office, upon payment of a fee of four hundred and twenty-five dollars any mortgage (including a revolving credit mortgage) on a vessel which recites the interest of the mortgagor in the vessel and the interest so mortgaged, provided it has been previously acknowledged, and provided further that written proof is furnished to him of the amounts and dates of any documents or evidence of debts in support thereof. At the time of recording the Deputy Commissioner will, if requested, certify without charge 2 copies of any mortgage so recorded.