The normal rate of contribution otherwise established under this article for a local safety member whose retirement allowance is determined under Section 21362, 21362.2, 21363.1, or 21366, and reduced under Section 21367 because his or her service is included in the federal system, shall be reduced by one-third as applied to compensation not exceeding four hundred dollars ($400) for services rendered in any month after the date of execution of the modification of the federal-state agreement, including services in the federal system, or the effective date of the contract or contract amendment pursuant to which a contracting agency and its employees become subject to this section, whichever is later, and prior to the date upon which services of persons in his or her employment cease to be covered under the federal system.