David,

Nice find. I have only seen this one time before and that is on the $100 from Isle of Capri in Biloxi. Here is a link:

http://www.chipguide.com/01470.jpg

Besides the large bar code on the left side, it has one of these "peepholes" on the right side, just right of the "S" in "MS".

I am sure it is a security feature of some sort.

Greg