The page you cite shows G.E. bulbs which were narrower near the base compared to NOMA bulbs. Westinghouse bulbs were also made in the same shape. As Tim mentions A refers to the standard "pear shape" bulb envelope - these are used in bubble lights, while C refers to the cone shape.
The bulbs you buy today aren't really cone or pear shape they're more like oval shaped.
The G.E. light strings from 1960s with metal clips would only accept G.E. or Westinghouse brand bulbs. Other brands wouldn't make contact in the socket due to the wider shape of the bulb. I don't know if this was on purpose or by accident.