Interesting Colt heavy barrel
This Colt heavy barrel came with an early Colt upper without the forward assist. A dealer at a gun show had three of these heavy barreled Colt uppers at a gunshow back around 1990, the cost was $200 each. I bought one and used a SP1 lower to test fire at 100 yards, it grouped just like an issue barrel, nothing special. The twist was 1 in 12.
Later I traded it to a friend but took these photos for later identificationAttachment 17485Attachment 17487Attachment 17486
Colt heavy barrel and early upper
Jeff, I was able to check this early upper and heavy barrel over this weekend. The upper has the Colt triangle and square C in front of the ejection port, no FW or brass deflector. The ejection port cover looks just like the 604. The slip ring is flat. The rear sight drum has only an arrow R.
The barrel is .680 at the muzzle, .715 in front of the front sight base and .750 between the bayonet lug and swivel. I checked the barrel again, it is 1X12 twist. The barrel has the exact profile as your photo of the Model 606 profile. The handguards are the glossy black triangular.
I have plans to get this upper/heavy barrel back as I think it is kind of a rare item. Thanks for your help and posting the photo of the 606 barrel profile