Het pied belly markers?

I was wondering what exactly do the " het pied belly markers" look like? I've seen them described as train tracks but can't seem to figure out exactly what they mean. If anyone has pictures ( or a link) comparing a normal belly to one with the het pied belly markers, it would be most helpful. Thanks!

I was thinking about this earlier. I think this would work well as a quiz. Post a mix of hets and normals. Then give the answers

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