I recently acquired one of these. It is a 1952 Proof Quarter that has an 'S' on the chest of the Eagle. Mischief at the mint . This one is slabbed by NGC.
It takes magnification to see it well and I am not that good at photography. Here is a link to an image http://hermes.csd.net/~coneca/content/InTheNews.htm
Here is a pic on the NGC slab with the "SuperBird" variety identified. Not sure why they did not note the FS ID.