One of my odd coin collections is Japanese occupation of China from about 1933 to 1945. Many of the aluminum coins have strange raised dots, often in a string. EXAMPLE: While searching for a few I don't have yet I found a reference to what the cause probably is. Rusted dies! I know that can be a problem on some early US copper coins but not to this extent. I believe the mystery is solved.
Can you show us more pictures of other coins with this effect? There are quite a few US coins struck from rusted dies, and usually it appears as a rough patch. The lines are odd - but it may be from the type or quality of metal used, storage conditions of the dies, some attempt at removing the rust, or any number of other causes. Very interesting!
