Are there any markings on  6520 or 6721 upper  ass'ys that help ID them as Colt or otherwise?

There should be a "C" forge mark behind the brass deflector and it's usually followed by the forging manufacturer symbol.

The sample picture shows that the forging was from Alcoa.

Check out more upper receiver forging markings in this thread link.

Thanks, Alpha-Romeo3.
I knew the info on lowers was there, but  didnt realize it had upper info also!

The 6520 is an A2 receiver, IIRC, and it could also be marked "C M" or "C MB".

My AR6520 was made in June 2005 and it has only a Keyhole looking mark.
I think that is the mark that Cerro forging uses.

Quoted: My AR6520 was made in June 2005 and it has only a Keyhole looking mark. I think that is the mark that Cerro forging uses.

Does it have the properly anodized extended ramps in the receiver?

Cerro Fabricated Products

 Great info. in that linked thread, but I've got a 'mystery' upper. Nam era configuration-forward assist but no brass deflector with the original style sights-but it is a 'big hole' upper. It's marked 'C  H' in the indent below the rear sight, right side. There are 2 light/small stamped marks slightly above and forward of the dust cover near the barrel juncture. It should be a Colt since that's all I ever had until the last 10 yrs or so. It doesn't match any of the markings on upper receivers that I've seen however.
 Wish I could do a pic. Ghost

Quoted:  Great info. in that linked thread, but I've got a 'mystery' upper. Nam era configuration-forward assist but no brass deflector with the original style sights-but it is a 'big hole' upper. It's marked 'C  H' in the indent below the rear sight, right side. There are 2 light/small stamped marks slightly above and forward of the dust cover near the barrel juncture. It should be a Colt since that's all I ever had until the last 10 yrs or so. It doesn't match any of the markings on upper receivers that I've seen however.  Wish I could do a pic. Ghost

CH uppers are one of the forgings Colt's was using on the M16A1 in the mid 70's and early 80's.

Quoted:  Great info. in that linked thread, but I've got a 'mystery' upper. Nam era configuration-forward assist but no brass deflector with the original style sights-but it is a 'big hole' upper. It's marked 'C  H' in the indent below the rear sight, right side. There are 2 light/small stamped marks slightly above and forward of the dust cover near the barrel juncture. It should be a Colt since that's all I ever had until the last 10 yrs or so. It doesn't match any of the markings on upper receivers that I've seen however.  Wish I could do a pic. Ghost

Nothing mysterious about it

It's a later type Colt upper (early "Sporter", but post SP1s and 2s), where they used up the last of their A1 type forgings right after the Armed Forces went to the A2.

Colt has always used up leftover forgings on the civilian pieces, when the armed Forces went to something new. That's why the SP1s and 2s are totally flat sided. They were built out of the last of the forgings they had for the very first Air Force contract.

Folks who have an early flat side lower, like me, and want to build an M16A1 or XM177E2 lookalike, without having to use an adapter pin, are often on the lookout for these big hole, forward assist, uppers.  The last one I saw for sale, by Pete in NH, had a $175 price tag; which I think was a bit high.  Got my own, at an Eastman show, from one of the parts guys there, for $90. That was a couple of years ago though. Like yours, it's marked C H.

If you have no use for the thing, or want to swap it for a small hole upper, I expect you could find someone on the EE section to take it off your hands.  Alternatively, Bushmaster has a sleeve pack to use the thing w/a standard small hole lower.

Ooops, missed that part about it being big hole.

 Thanks, gents-that was quick! It all makes sense timewise as well. It's got a Colt govt. profile [newer?]1:7 bbl., 20" on it at present. Don't remember putting it together so I probably just pulled it off an AR to make a Carbine. It's been a safe queen for years.
 Good suggestion on the EE board-might swap it for a flat top to build a rifle for scope use. Ghost