Just Machine Doubling. RPMs ended in 1989. After 1990 the mint mark was part of the dies design and the dies were created at the Denver location at that time. Just a sever case of Machine Doubling that altered the mint mark. (you can also see it on the '7' as well)
The lines on the top of the mint mark are call metal striation lines. When metal slides over another piece it will leave these lines. If you look a stick of warm butter and slid the edge of a paring knife lightly over it, you will see the lines on the edge of the knife. We often see these from die movement.
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