I've never heard of that before. That being said, the mm on your coin looks perfectly within the norm for a circulated 1977D cent.
There's also another error listed for the 1977d the o in of on the reverse is a lowe case o, have you heard of that, loo MI ng at my coin it looks like a lower case o. Both these errors I quoting are on sale on etsy now for considerable money
Etsy is not a reputable place to look for coins or see actual values for coins. Also, what book are you getting your information from?
Of course if you look at ANY Memorial reverse Lincoln cent you'll find that they all have lowercase O's. By the way, so do Franklin half dollars.
That's a new one on me and I've been collecting for over 55 years. The correct term would be "filled D."
