The A1 sight, shell deflector equipped upper was created essentially for the Canadian M16, the C7. The Canadians didn't want the A2 sights on their version. Colt did sell some AR15s with this type upper as well.
Given that the markings on your barrel (C MP B) indicate an early, fully chrome lined piece, I would suspect that it's an aftermarket put together; since the upper would have been made several years after the barrel. Later chromed A1 barrels are actually marked "chrome bore".
The D K markings I'm not positive about. "K" should be Kaiser, as producer of the raw forging, but a Colt piece should be marked "C K". The "D" may stand for "Diemaco", the Canadian outfit that makes the C7. Currently, Diemaco uses their corporate emblem as an ID mark, but perhaps they used the "D" on early production pieces.
BTW, though you use the word "cast" in you note, be aware that nothing on your piece is a casting. All strictly forgings