Yes. Those are machining marks that assist in balancing the slide weight. A fine tuning if you will.
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I am going to get my drill press set up and start balancing my Berettas, Kahrs, Para Ordnance, Ruger, Makarov, and even a left over Kel TEC. I just wish I knew how to determine how the slides are out of balance and where I drill how many dimples to fix them.
Nazca Lines?