4. Black Diamond Gem briquettes, manufactured by Coal Products Limited at Immingham Briquetting Works, Immingham, North East Lincolnshire, which—
(a)comprise anthracite duff (as to 20 to 30 per cent of the total weight), petroleum coke (as to 40 to 45 per cent of the total weight), bituminous coal (as to 12 to 22 per cent of the total weight) and molasses and phosphoric acid binder (as to the remaining weight);
(b)were manufactured from those constituents by a process involving roll-pressing and heat treatment at about 300° C;
(c)are pillow-shaped briquettes marked with two parallel indented lines running latitudinally around the briquette and have an average weight of 160 grammes; and
(d)have a sulphur content not exceeding 1.5 per cent of the total weight.