This is an old thread. However, if it looks like the coin in the first post, it's a plating blister or the plating has been breached and the zinc is starting to corrode under the copper. It's very common to see this on copper plated zinc cents. There some good information on the 1st page of this thread explaining why plating blisters occur.
The person you quoted has not been on CoinTalk since July 2019 (thank goodness) David.. Start a fresh new thread. Pictures are needed. It's not a RPM anyways because the mint marks were no longer hand punched onto the Die starting 1989. There are no RPM's after that year.
Wow I was wondering how many rounds this was going to take before somebody got knocked out LOL with that check these out I know I should have made a new post but this coincides with this one