I looked it up. I can look up the date when the USDA "suggested" the flour to be enriched. This is from http://www.dietitian.com/foodlab.html
The following wheat flours do not include red durum or semolina, which contains bran specs from durum and both are used in the manufacture of pasta. Nor are soft wheat flours included, which are used in the manufacture of cakes, cookies and pastries. Bread flours are made from hard wheat, which contains more gluten (protein) and provides structure to bread.
Wheat Flour is made by grinding wheat and typically does not contain the bran or germ.
Enriched Flour (wheat) is wheat flour (no bran or germ) that has been enriched with thiamin, riboflavin and niacin and may include Vitamin D, iron and calcium that are lost during flour processing.
Enriched Flour (flour) is the same as the above except that the source of the grain is not identified. Other grains such as rye, oats, barley or soybeans can be used in the making of this flour.