2015 Cents are Copper Plated Zinc Planchets. It's a plating issue on the L called a Plating Bubble. Not a Mint Error.
FYI - Mint Marks are part of the Die in 2015.. No longer hand punched into the Die since 1989. So you can't have a RPM anywhere on the Cent.
I explained it in my first post. Plating issue. Copper plated zinc planchets suck. The bubbles or blisters form because the plating is affected by air trapped under the copper or environmental damage.