(a) workmen employed in any employment or class of employment which is, or has been declared to be, an essential service under rule 126-AA of the [Defence of India Rules, 1962, or under rule 119 of the Defence of India Rules, 1971;]

(b) workmen employed in any factory as defined in clause (m) of section 2 of the Factories Act, 1948 (63 of 1948);

(c) workmen employed in any mine within the meaning of the Mines Act, 1952 (35 of 1952);

(d) workmen employed in any major port;

(e) workmen employed in any plantation as defined in clause (f) of section 2 of the Plantations Labour Act, 1951 (69 of 1951);

(f) workmen employed in any employment specified in this behalf by the Central Government by notification.