The periodical payment to the Crown, in respect of the produce of a mine which by the Royal Mines Act 1424 (c.12) belongs to the Crown, of an amount which is not fixed but is calculated as a proportion of that produce is not—
(a)a payment to the Crown qua superior for the purposes of section 54 of this Act;
(b)a perpetual periodical payment for the purposes of section 56 of this Act; or
(c)a feuduty for the purposes of Part 3 of this Act.