Can you all check this out? Has anyone ever seen one of these? I didn't notice it until I took a photo to put into my coin collecting software.
It would be hard for me to say wether it is post mint damage or not but there is plenty of experience here so maybe they will be able to help you out. Good luck.
What I can tell from the pic it looks like Machine Doubling, but without a better pic I can not say for sure. Maybe post damage but I would put my money on Machine Doubling because you can see some MD around the two 5's.
http://www.coppercoins.com/advsearch.php type in 1955 to 1955 and select San Francisco and hit search and you can see quite a few varieties on the 1955-S cents to compare[SIZE=-1][/SIZE]