Those with heat rings and no markings whatsoever (?-1939) are older than smooth bottom without any markings (1939-?). Neither may have been regular production, possibly just made to order in quantity.
Next are those which say "6-1/2 INCH SKILLET" or similar, followed by those that also have "MADE IN USA". Both of those all appear to have the "late" style Wagner handle, so ca. late 1950s or after (with MADE IN USA probably mid-1960s and after).