For a while, Colt was installing a steel block in the lower with 2, and later 3 blind-pins. This was to prevent the installation of either a USGI-type auto-sear OR a Drop-In Auto Sear (DIAS). Essentially, it was strictly a "PC" move on their part.
[img]photos.ar15.com/ImageGallery/Attachments/DownloadAttach.asp?iImageUnq=9698[/img]
Colt "Sear-Blocked" lower
[img]photos.ar15.com/ImageGallery/Attachments/DownloadAttach.asp?iImageUnq=9700[/img]
M16 lower with auto-sear for comparison.
An unblocked lower is definitely more desireable. The block not only prevents the use of standard carriers, but also prevents some aftermarket triggers from being installed. It *is* legal to remove the block, but you'd want that done by someone with experience doing so, and have the pin holes filled in.
-Troy