Your efforts are exemplary and your findings profound. I commend and applaud your approach to overlays. Your inquiry using this method, especially year-by-year, may substantiate some hunches we both have as to how and why date punches were made, used, misused, altered, and destroyed.
These mysteries must be the most interesting across all US coin series. Never lose faith in the activity on this forum; we are setting the bar for collaborative and constructive research into our most precious hobby.
Collecting Flying Eagle and Indian Head pennies never made more cents!