In lieu of the authorizations or requirements provided for in Sections 24920 and 24922, the plan may provide only that the retirement salary shall be based on service rendered prior to the effective date of the plan or prior to July 1, 1944, and shall be an amount which, when added to the retirement allowance the respective member is entitled to receive under the State Teachers’ Retirement System, shall equal a sum of not less than one hundred dollars ($100) per month and Sections 24920 and 24922 shall not be applicable thereto. The costs of the benefits under such a plan may be met by the contributions of the districts alone, notwithstanding Sections 35161, 35162, Article 1 (commencing with Section 7000) of Chapter 1 of Part 5, Article 2 (commencing with Section 10010) of Chapter 1 of Part 7, Article 1 (commencing with Section 12500) of Chapter 5 of Part 8, this part, Article 5 (commencing with Section 32340) of Chapter 3 of Part 19, and Part 25 (commencing with Section 44000), and only teachers who have credit for service rendered prior to that date shall be subject to the plan.