Commencing October 1, 2005, any pensioner or annuitant who retired prior to October 1, 2004, from a city, town, county, or public or quasi-public organization of the state before the city, town, county, or public or quasi-public organization of the state became a member of the Employees’ Retirement System and is receiving a monthly benefit not administered by the Employees’ Retirement System shall receive an increase of four percent of their gross monthly benefit, but not less than fifteen dollars ($15) per month, provided the city, town, county, or public or quasi-public organization of the state elects to fund and pay the increase.