the scope mount replaces the top cover - it slides in from the back. the screws tighten little clamps/plates which secure it to the receiver:
http://www.dsarms.com/prodinfo.asp?number=620APARA

here's the handguard:
http://www.dsarms.com/prodinfo.asp?number=US021FAS

it looks like the one in the TD ad because it IS the one in the TD ad