Maxm: You nailed it - Fighting Iron by Art Gogan.
Page 100 with illustrations on Page 101.
Quoting from Page 100 - "The distortion of the metal grains in the hot forging process is evident as an elongation of the grains along the axes that are parallel to the path of the plastic-like flow of the hot metal under the influence of pressure applied by the hammer or the die(s). These severely elongated grains result in residual stress gradients between the distorted grains and their boundaries or interfaces with adjacent grains. Because some corrosion mechanisms display differences in the corrosion rate that are attributable to the presence of stress, older forgings that have experienced slow, uniform surface corrosion will frequently exhibit a pronounced visibility of the "forging flow lines" as the corrosion highlights the elongated granular structure of the forging (see photos on right-hand page). These gently curving, striated surface features will often be observed on the well aged and worn ferrous forgings commonly used for firearm frames and receivers of the last half of the 19th century and first quarter of the 20th centuries. Unquote.
