It looks like micro engraving but don't know. Could be a strike through debris. I've never seen this before
PMD It was pressed against something with some letters Or it could just be pareidolia Not a mint error...
Odd mark for sure. Thanks for posting full image, but even enlarging it more on my laptop, I can't make out the letters, but it's more than the reeded edge of another coin. I'm with the others on PMD though.