3If in any case the Board think it desirable so to do for the purpose of protecting the interests of persons entitled to any mortgage, charge, or other incumbrance on an annuity, they may direct that the certificate to be issued in respect of the annuity shall be issued to and held by such persons as they appoint to be trustees for the purpose, and the persons so appointed shall, subject to the provisions of any regulations made under this schedule, be deemed to be the persons entitled to the .annuity.