Couple questions about this CH marked A1 upper. One, it lacks the triangle and square C proof marks. .?? Also, what do you think made these marks. They are on both sides, but more noticeable on the left side. I can only assume some type of barreling tool. I'm also guessing the receiver was refinished, because the marks did not remove any finish. Any opinions would be much appreciated.

I don't know what made them but know of an SP1 carbine(1984 vintage) original owner, that as the same marks. I know as fact it was never refinished. Came from factory that way.

Not uncommon with factory Colt rifles especially M-16's but as stated above occurs on original SP-1's also.  Some think it's during de-mill or barrel change but not the case generally.  Have taken receivers out of cage pack that had them also.  Not sure if holding fixture before finish is applied as most are always under the finish or if done when barrel is installed and finish is simply compressed.  First one I ever saw I thought I had done it installing a barrel but then checked a couple receivers that were factory fresh and they had the same markings.  Kinda gave me a sigh of relief.

Thanks for the info. Nice to know finish may be original, then. And the lack of proof marks?

I have an SP1 upper with those marks.

Generally lack of proofs means replacement parts and not a proof tested factory assembled rifle.  It can also mean refinished and finish simply covering the proofs.  Only way to tell is strip it so kinda mute if finish isn't already trashed requiring stripping if one actually cares.  Did upper come in one of the de-mill kits or picked up singularly?  I haven't seen any of the de-mill kits that were arsenal refinished or lacked proofs but have seen where some of the small parts had been replaced with non Colt whether from foreign military in service purchase / repair or more than likely vendor purchase / replacement to overcome de-mill shortages.

Thanks again. Upper was picked up by itself, so don't know its history. Lack of proofs doesn't bother me. Was just curious. I appreciate all the replys.