Here is my analysis. I welcome everyone's polite and respectful input, at least until Scott Ryan chimes in and then that will settle the matter.
My conclusion is that this is a very early Colt 733, part of the famously hard to identify "transitional" 700 series when Colt seemed to put together rifles with whatever parts were on top of the parts pile. This would be the first time I have seen a 733 that was not stamped M16A2 (whether on an A1 or A2 lower or not).
You decide. If I have misidentified any aspects below please correct me. Click on each picture for a HIGH resolution photo. If additional photos are needed to make a definitive identification ask and ye shall receive.
BUFFER: Standard Carbine Buffer.
SPRING: Standard Carbine Spring.
BUTTSTOCK: 3rd Generation Collapsing Stock. Factory staked in place with varnish over staking.
BUTTSTOCK MATERIAL: Fiberlite.
TUBE: 4-Position Tube.
LOWER RECEIVER: Full Fence.
PROOF MARKS / FORGE CODE: Colt Forge Mark inside Carry Handle below Rear Sight.
PROOF MARKS / FORGE CODE: Colt Forge Mark on Front Site.
REAR SIGHT: Standard "Late Production" A1 Sight.
EJECTION PORT COVER: "Late A1 Production" Type.
FORWARD ASSIST: "Original" Tear Drop Assist.
BOLT GROUP: Standard Colt Bolt Group with a "C" marked Carrier, and a "MPC" marked Bolt.
HANDGUARD SLIP RING: Tapered.
BARREL DIAMETER: 0.625" at the Front Sight Base.
BARREL TWIST RATE: Do not know. Probably 1/7.
BARREL MARKINGS: C MP CB.
HANDGUARDS: Carbine Handguard with Single Heat Shield.