I'm building a spare upper so I don't have to shoot my vintage upper that I only paid $350 for 5 years ago. My 604 has the D stock and notched parked carrier so I will leave it as is. Im glad because I like the D stock better than a trap door and the bolt must be head spaced right because it is very accurate.
My early unfired Sp1 had a 3 prong flash hider. I should have never got rid of it. I will be using a like new SP1 upper, like new hand-guards, a green mountain 1/12 a1 barrel, and a DPMS 3 prong I have had for a while. I also have a slick slide chrome carrier I will probably use just because I like to look of it unless and I can find a slick side parked bcg.
SP1 Large hole upper
C H forge
C in square and Triangle stamp
NOS 604 complete upper
No forge markings
C in Square and Triangle stamp
a1 birdcage flash hider
C MP C barrel
My Colt 723 clone
No forge
C i square and triangle stamp