1970 County Group Maps
County group areas identify economically related groups of counties. They are subdivided to identify SMSA's and county components thereof with more then 250,000 population , and other subareas occur as remainders of larger units, are designated by the term "Part" with the subarea number.
The subarea indicated in the example is elsewhere designated by the five-digit number 01609: the first three digits identifying the area, the last two digits identifying the subarea. Where there is no subdivision of an area, a subarea code of 01 may be assumed.