No provision of this act shall apply or be construed as having heretofore applied to any city incorporated or reincorporated under Act No. 279 of the Public Acts of 1909, as amended, being section 117.1 to 117.38 of the Compiled Laws of 1948, unless specifically adopted by the electors as a part of its city charter. The provisions of this act shall be deemed to be in the nature of charter provisions for any city incorporated thereunder or subject thereto and any such provision may be altered or removed by amendment adopted by the electors as provided in Act No. 279 of the Public Acts of 1909, as amended, provided that the effect of the amendment is such as might legally be accomplished by charter provision in the case of a city operating under said Act No. 279. All such amendments heretofore so adopted by any city incorporated under or subject to this act, and all actions heretofore taken and all bonds heretofore issued under or in accordance with such amendments, are hereby validated to the same extent as if the foregoing provision had been in effect when such amendments were adopted.