In all cases where a city containing over three hundred thousand inhabitants is co-extensive in boundaries with the county, all of the officers known therein as city treasurer, city controller, city commissioners, shall severally be regarded as county officers, and as such shall severally be subject to all the provisions of this act, and shall have all the powers and perform all the duties herein enjoined, and be subject to the same penalties as if they had each been elected or appointed as county officers, and had been designated as such.