(A) Upon retirement for disability on or after July 1, 1976, a Class One member shall receive a service retirement allowance if he has attained the age of sixty-five years. Otherwise he shall receive a disability retirement allowance which shall be computed as follows:

1. (1) Such allowance shall be equal to the service retirement allowance which would have been payable had he continued in service to age sixty-five based on the average final compensation, minus the actuarial equivalent of the contribution the member would have made during such continued service, with an interest rate of four percent per annum.
2. (2) Notwithstanding the foregoing provisions, any Class One member whose creditable service commenced prior to July 1, 1976, shall receive not less than the benefit which would have been provided by the provisions of this section in effect immediately prior to July 1, 1976.

(B) Upon retirement for disability on or after May 19, 1973, a Class Two member shall receive a service retirement allowance if he has attained the age of sixty-five years. Otherwise he shall receive a disability retirement allowance which shall be computed as follows:

1. (1) Such allowance shall be equal to the service retirement allowance which would have been payable had he continued in service to age sixty-five based on the average final compensation, minus the actuarial equivalent of the contribution the member would have made during such continued service, with an interest rate of four percent per annum.
2. (2) Notwithstanding the foregoing provisions, any Class Two member whose creditable service commenced prior to July 1, 1964, shall receive not less than the benefit provided by subsection (A) of this section.