The mark on the '6' is on all 1960 small date cents:

It was a re-engraving mark on the die. Also the throat area was added at that time as well.
Why is this not Machine Doubling? It looks like it. If it were
Machine Doubling, all the other areas of the design on that side of the other devices would all be showing the same thing. Back then they were having issues with the master hub that created the new dies with a swelling issue. Through out the years, different enhancements were added to the master hub/master dies to enhance the devices a bit more. Thus we see the same issues for many year on the 'WE' on the motto and other devices as well:


Other years had a single year of enhancing the devices (Similar to a face lift on the devices on the master dies.):




There are other cases probably on many other denominations as well. Some master hubs/dies had issues that were fixed through the years. When we see a continuing issue on many years it was an alteration of the master hub. If it was for just one year, it was altered on the master die.

Thanks to Will Brooks we have this information on cents. But knowing this information helps us understand easier when we see these differences. These are things that true collectors notice. So I'm sharing this information with you.
CoopHome:
Does the mint alter a master hub or master die? How do we know if it is a master hub or a master die alteration?
Edited by coop
06/03/2022 12:25 pm