For those zoning districts which do not have sign regulations as a part of the zone provisions, and for those which are not readily placed into classifications referred to in preceding sections, the Planning Commission shall classify said zones as either: residential (R), agricultural (A), commercial (C), shopping center (SC), or industrial (M) zones depending upon the similarity of the characteristics and permitted uses of said zone to those already classified. When such a classification has been made by the Planning Commission, the sign provisions applying to the respective classification shall apply to said zones.