Mods, sorry if this is in the wrong section.

I got this with a Sendra RR machine gun a while back. I know you have to keep your original barrel length but that's why I may trade it for an A2 clone (Everything accept the lower).  I tried to get pictures of the lettering printed on the barrel but couldn't The barrel is marked C MP Chrome Lined.
Basically I just want to make sure this isn't worth much more than a standard A2 before I try to make a trade.

Who ever trades you for their complete A2 upper will come out ahead. Original A1 parts do very well for themselves.

The barrel would be Colt. No question. The Delta ring is not correct for that barrel assembly. It sghould have the flat slip ring. The upper manufacturer is hard to determine by the pics. The port door in not correct as it is an A2 type. The Stock, pistol, and handgurds look to be in nice condition. The barrel and furniture is where the money is. Not knowing who the upper is made by leaves it in question. In my opinion, The upper assembly (including BCG and CH), stock, and pistol grip would be valued somewhere around $550-$600. What you have is a mixmaster with some original M16A1 or SP1 parts (barrel, handguards, stock, and pistol grip).

There is nothing in federal law that makes you keep the original barrel length, overall length, or caliber. Those characteristics are the state it was in when it was transferred. Many people keep the parts to restore the weapon to the configuration listed on their Form 4, so the next Form 4 will have the same listed configuration, but there is nothing that requires the configuration to stay the same. If you sold your 20" upper, and replaced it with an 11.5" upper, then decided to sell your MG to another individual in your state, his Form 4 would list  an 11.5" barrel length,  and a corresponding change in OAL. ATF maycontact you to verify the charges, but no big deal.

The value of your MG has very little to do with its current configuration. Its value is mostly due to being a transferable MG.
Factory Colt MGs probably benefit from being in the original configuration, with correct parts.

Interestingly enough, I remember reading BATFE guidelines and for a firearm to be considered C&R (a consideration because in the not so distant future all MGs will be 50 years old or older) at 50 years old it has to be in ORIGINAL configuration.  Perhaps this is a stupid BATFE rule, im not familiar with the actual legislation on this as written, but I think it’s a good idea to keep the original upper to keep its “original” configuration.

There is a Retro section in the AR15 topic list.
You'll get more hits there.

I had this over in the AR15 area. The mods moved it here for some reason.

I haven't found any marks on the upper at all. The upper would have a "C" on the carry handle if it were a Colt wouldn't it?

I have a 1978 SP-1 I purchased after HS graduation back then.
Next time I'm in the safe I'll take a look at, may be a day or so, just home from hospital.

It’s a later “round forge” 603 upper. To try and tell who made it I need a photo of the right side under the rear sight. You said no forge codes and with it being round forge it should if Colt.

It’s a later Colt barrel

Delta ring rather than slip ring and later A2 port door

Earlier type D stock

Earlier improved 3 prong flash hider

Did not leave Colt as a factory built upper as it doesn’t have proof marks

Need pics of bolt and carrier

Furniture looks nice but broken tooth on the hand guard hurts.

I would say realistically if I were buying it would be around $600 as its value lies in the barrel

FWIW,

I sold a similar rifle, not in as good shape, for $1K last weekend,

M16A1 upper, stock and grip
very early DPMS  lower with a generic LPK and BCG,

the parts kit was complete but the handguards were not as nice as what is pictured,