49HC Oil Derricks, prairie green glaze; 4/2 hole set, brick red Sapulpa clay. Frankoma started to use this well define plug recess on the derricks in 1950. The earlier shakers have a rounded plug recess. Frankoma shakers had natural cork plugs until about 1970 when they switched to a red rubber plug. About 2000 they switched again, this time to a white plug.