If any municipal corporation has authorized bonds to bear interest at a rate of interest not in excess of six percent prior to March 5, 1970, and any of such bonds remain unsold on that date, the legislative branch of the municipal corporation may approve an increase in the rate of interest of such authorized but unsold bonds and the increase in such rate of interest is hereby declared to be legal and valid, and such municipal corporation may issue such bonds which shall bear interest at the rate of interest approved by the legislative branch which increased rate of interest is declared to be legal and valid. (Added 1969, No. 177 (Adj. Sess.), § 2, eff. March 5, 1970.)