It looks to me like what we call 'worm corrosion'. A sort of thread pattern corrosion that forms from within the steel in that worm/thread pattern. It's almost like the fine corrosion thread pattern that you'll find threading its way under the paint on your car. One of the metallurgists (JM......) can tell us the technicalities of what it is. I say this because nothing about the marks is regular in a way that, say, the figure 7 is in shape, depth and thickness.

You would often find this worm corrosion on rifle bodies that have been painted over