The directors may, if they think fit, from a shareholder willing to advance the same, all or any part of the moneys uncalled upon any shares held by him or her, and upon all or any or the moneys so advanced (until the same would, but for such advance, become presently payable) pay interest at such rate [not exceeding, without the sanction of the Bank in general meeting, six per centum (6%) per annum] as may be agreed upon by the member paying the sum in advance and the directors. If the whole amount unpaid on any shares is paid, the directors may issue those shares as fully paid up.