1976 S Cent Plating Error In Sealed Bicentennial Mint Set

Hi

I am new to the community and thanks for allowing me to part of this valuable community. My Grandfather was a coin collector and passed in 1992 and left his collection to my dad who was also a collector. My dad passed last June. My Mom asked me to go through the hoards of coins. I am a novice, but good with internet research.

I came across a sealed 1976 Bicentennial US Mint set (one of many) that has a 1 cent 1976 "S" in the unopened set.

What caught my eye was the unusual plating on the front of the coin. Almost looks like an eclipse of mostly a thin layer copper and the last 1/3 exposing the tin/zinc.

Is this common? I have read a few postings online with similar odd errors, but each one was questionable coming from circulation or from an unsealed set.

I did not want to remove the coin from the proof case to weight it just in case.

If this is just a worthless error, I can accept that, but if something to pursue, I equally would accept that as well.

Thank you in advance for your insights.