Nice find... Now those that don't know about the Floating Roof will be curious and want to start looking for them
The mint polished off the low relief design. Sometimes because of a clash, or to remove a die crack, on an otherwise usable die.
Found this https://www.coincommunity.com/forum/topic.asp?TOPIC_ID=236773 One person said that this is also common for 1970 Lincoln memorial cents.
So close to floating! It was somewhat polished but if you look closely you can still see the ghost of the vertical lines. Thanks for making me look closely.