The variety inverted "S" occurred on stamps 20, 65 and 70 on a sheet of a hundred. 25 stamps were surcharged at a time. The stamp on the right has the error. But also note the thinner numeral and letters. The letter "e" also seems to form a complete circle.
Here we have a mint block of 4 with the top right stamp having the inverted "S". Lastly, there is also a variety of the surcharge where the letter "S" has fallen off and was later inserted by hand. This exist for both the 4c and 10c surcharges. They are very rare and I have yet to have sight of one.
But the most valuable stamp of the group is the 2c double surcharge. I have seen one stamp so far but it looks more like an offset rather than a true double.