Actually, I think by that date the MM was added to the die, so this would be impossible. Wait for someone to correct me though...
Welcome to CT. The Mint no longer hand places Mint marks on the master dies. The Mint had converted over to placing the mint mark on the model for making the master hubs of business strikes by 1990. They stopped the placing of the mint mark by hand on proof hubs in 1985/ 1986. No over punched mint marks anymore.
Most likely a stain or metal anomaly. It is not a small mint mark. It is not a mint error and is is not a variety.