19.—(1) Where the consideration, or any part of the consideration, for a conveyance on sale consists of money payable periodically for a definite period not exceeding 20 years, so that the total amount to be paid can be previously ascertained, the conveyance is chargeable in respect of that consideration with ad valorem duty on the total amount.
(2) Where the consideration, or any part of the consideration, for a conveyance on sale consists of money payable periodically —
(a)
for a definite period exceeding 20 years or in perpetuity; or
(b)
for any indefinite period not terminable with life,
the conveyance is chargeable in respect of that consideration with ad valorem duty on the total amount which will or may, according to the terms of sale, be payable during the period of 20 years next after the day of the date of the instrument.
(3) Where the consideration, or any part of the consideration, for a conveyance on sale consists of money payable periodically during any life or lives, the conveyance is chargeable in respect of that consideration with ad valorem duty on the amount which will or may, according to the terms of sale, be payable during the period of 12 years next after the day of the date of the instrument.