A few of the "clear font over black foot" Corinthians have been found to contain labels that were inserted between the font and the foot. Evidently they were inserted at the time the two parts were glued together using the metal connector. Only a small percentage of these lamps do have such a label and I have not found them on the other colors of Corinthian lamps.
Another interesting note to this particular lamp is that while it is commonly referred to as "clear over black" the foot itself is not always black. It can be dark green, dark blue, or dark red as well. The glass however is so dark in all the colors as to appear black. It takes a high intensity light to actually determine the true color of the foot. The variance in color does not effect the collector value of the lamp itself.