(

a) Notwithstanding any other provisions of this chapter, every retirement allowance payable for time commencing on the effective date of this

section to any previously retired member of a superseded system not established pursuant to either

Chapter or

Chapter is hereby increased, by increase of the pension portion thereof, to the amount it would be if the previously retired member of such superseded system had been retired under the provisions of this chapter, and the provisions of this chapter, as they are in effect on the effective date of this section, had been in effect at the time of the retirement of the previously retired member, assuming that all of the service with which he was credited at the time of his actual retirement constituted prior service under this chapter. (

b) However, if such retirement allowance payable for time after the effective date of this section, as increased by subdivision (

a) of this section, is less than one thousand two hundred dollars ($1,200) a year, and if the previously retired member of the superseded system (not established pursuant to either

Chapter or

Chapter 5) was credited at the time of his retirement with or more years of service, or was retired after attaining the compulsory age of retirement, an additional amount provided by contributions of the county shall be added to his retirement allowance which will cause his total retirement allowance to amount to one thousand two hundred dollars ($1,200) a year. (

c) This

section does not authorize any decrease in any such retirement allowance, nor does this

section give any such previously retired member of such superseded retirement system, or his successors in interest, any claim against the county for any increase in any retirement allowance paid or payable for time prior to its effective date. Calculations of retirement allowances under this

section shall be made on the basis of current interest rate and mortality tables.