In relation to an agreement which, on the appointed day, becomes subject to registration under Part I of the [1956 c. 68.] Restrictive Trade Practices Act 1956 by reason of its having effect as from that day as if the Post Office had been a party thereto, section 10 of that Act (particulars to be furnished for registration) and section 7 of the [1968 c. 66.] Restrictive Trade Practices Act 1968 (consequences of failure to register) shall have effect with the substitution, for references to the time within which particulars are required to be furnished under section 6 of the last-mentioned Act (time for registration of agreements), of references to the period of three months beginning with that day and the said section 6 shall not apply.