Which article, introduced by Constitution's 28^th Amendment Act, empowers the Parliament to vary or revoke conditions of service of persons appointed to the Civil Service of the Crown appointed in India before the commencement of the Constitution?

According to Article 312a, Parliament may by law- vary or revoke, whether prospectively or retrospectively, the conditions of services as respects remuneration, leave and pension and the rights as respects disciplinary matters of persons who, having been appointed by the Secretary of State or Secretary of State in Council to a civil service of the Crown in India before the commencement of this Constitution, continue on and after the commencement of the Constitution (Twenty-eighth Amendment) Act, 1972, to serve under the Government of India or of a State in any service or post.