Difference in mil spec lowers??

[mmcbeat]

I am sure this has been addressed before but I could not find it. I am asking out of curiosity more than anything else. In the picture below, the lower on the left is a recent manufacture Colt 6920 and the one on the left is a new Noveske that I am building. In the receiver the Colt has a "wall" just above the selector where the Noveske does not. Purpose? Pros/cons? Thanks.
http://i61.photobucket.com/albums/h65/mmcbeat/lowers.jpg

[tactical1]

Block for autosear?

[Severian]

Yup, this is the Colt autosear block -- prevents the installation of burst and full-auto trigger groups. All the civilian Colts are like this.

[mmcbeat]

Thanks for the replies. That makes sense. Again, I was curious more than anything else. I have only recenty became interested in AR's. Learning a lot from this board. Kind of ironic that the block is on the "civilian" Colts that are marked Restricted/Military/Government/Export Use Only. Do any LEO's or Military actually use the 6920?