With respect to participants who shall belong to and retire under the supplemental pension system established by Subpart 1 after having accumulated 20 or more years of credited service thereunder but prior to his or her having accumulated 30 years of credited service hereunder, benefits payable hereunder shall commence on the date on which such participant would have accumulated 30 years of credited service hereunder had such participant not retired but rather had continued in his or her employment with the city, without interruption, as a firefighter or police officer. The annual benefit thereupon payable herefrom shall be an amount equal to that which would have been payable under Section 45-37A-51.220 had such participant not retired but rather had continued in employment with the city, without interruption, as a firefighter or police officer. For the purpose of computing benefits, in the determination of the firefighter’s or police officer’s final average salary, the firefighter or police officer shall be deemed to have received as compensation the salary which was the basis of payments the supplemental pension system made to the system established by this subpart from the date whereon the firefighter or police officer retired under the supplemental pension system to the date whereon he or she begins to receive the pension payable under this section.