I have an used upper that I've been trying to identify, after extensive searching online and this forum I decided to see if anyone has seen this.
There's the cardinal forge mark on the side of the upper receiver, but there's also an "M" marked on the front above where the gas tube/barrel attaches. I've read 'M' could mean Mueller Industries, but there's already the cardinal forge marking. So is the 'M' something added by the company who marketed the upper?
No markings on the front site post. It had an aftermarket barrel on it at purchase (several years ago).

I can get pictures if needed. Flat top.
Let me know if this has been hashed over, or goes elsewhere.
Thanks.

Pics would really help.

The Cardinal Forge mark doesn't really mean much.  It just means that they made the forging (the big block of aluminum raw material that in the vague shape of an upper receiver).  They have supplied all manner of companies.  It doesn't reveal the more important part: who machined it.

If I understand your post correctly, the "M" is right above where the gas tube enters the upper receiver.

According to this old thread, it was likely made by LAR, which has subcontracted for a number of companies:

https://www.ar15.com/archive/topic.html?b=3&f=12&t=230443

I have that M mark on the bottom of the uppers rear takedown lug with a dot impression im assuming the dot indent is where they tested for hardness ?

My Colt 6721 has 'M4' at that location. My CMMG flattop upper with a square forge mark has no mark on the front above the gas tube.

Ok, LAR might be it, because I have the T marking on the top rail. It doesn't have the extended M4 feed-ramps though. Has a nice finish (nicer than my bushmaster). And it appears LAR sells their uppers to several other manufacturers. Thanks for the help. Pictures attached it provides any more info.

https://lh3.googleusercontent.com/-RrmptiWfTI8/UwEpHG3WrbI/AAAAAAAAAq0/mbl2n107OGs/w709-h532-no/DSCN4960.JPG

https://lh4.googleusercontent.com/-t85nrdCA8Jo/UwEpImHJOEI/AAAAAAAAArA/GCnTT8ESWYc/w709-h532-no/DSCN4962.JPG

https://lh4.googleusercontent.com/-6d6V1sAWocA/UwEpJZX2eGI/AAAAAAAAArE/7gioMM0UR3U/w709-h532-no/DSCN4963.JPG

Yeah, I'm going with LAR.

It just means it has M4 cuts.