I was looking around Armslist.com and found a guy selling two EOTech's, a 551 and a 552. The price was too good so I checked it out. Not sure about the 552, but the 551 is definitely missing the optical component that rounds the top corners of the window.

Here's a link to Armslist.com classified ad.

http://www.armslist.com/posts/3675075/charleston-south-carolina-optics-for-sale--eotech-551---552