At the AUSA Expo a Colt Advanced Piston Carbine (APC) was on display with a new monolithic upper. Instead of the regular Colt APC quad rail, this version only has a picatinny rail along the top. It is tapped and drilled for picatinny rail sections, or other compatible accessories, on the sides and underneath. The rail extends to the end of the gas block and a rail extension can be bolted on allowing for an extra few inches of rail forward of the front sight/gas block.
Many thanks to Ten Pound Monkey for the photo.