PSA Slab Stress Marks? Or has slab been tampered with? 1959 Mantle PSA 6

I am thinking stress marks, but want to make sure. Bought on ebay a couple years ago and recently noticed the marks on slab.
Thanks
Brian

Thanks
Brian


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PS: nice 6!
Love the beckett holders. Saw a video of a guy dropping his bgs in the toilet then running it over with a truck...card was just fine