I have a few Colt upper receivers which are allegedly M4 uppers.  They all have the feed ramp cuts, which appear to be factory cuts with the proper finishes on them (and not done after the receivers left the factory).  But the differences are the markings above the gas tube hole.  Some have "M4", while others just have a "4".  No telling what comes from Colt, so I am just wondering whether these are all correct, despite differences in the markings?

Is there an online guide for what to look for in genuine Colt parts?

Thanks in advance.

They are factory correct despite the differences in the markings.  There were also some Colt A2 uppers that had the factory feedramps but did not have any markings over the cloverleaf.

"4" marked ones are older M4 uppers. These could be considered first generation.

"M4" marked ones are newer M4 uppers.

If its a Colt flattop, it'll have the "C AF" markings on it.