It is  a preban upper with a 16" barrel.  The seller believes it is a Colt upper because he bought it with some other Colt parts but he is not absolutely sure. Ther is a "M" stamped in front of the rear peep site and on the barrel it is stamped with a "SS"  and "5.56",  it also has a 5" flash supressor and bayonet lug.  It is a complete upper with bolt carrier, charging handle, etc.  My questions are:
How can I tell if this is a Colt upper?
What do you think this is worth?
Would this be a chrome lined barrel?
What would the twist be on this barrel?
Any information would be greatly appreciated!
Thanks!

Sounds like an Olympic Arms barrel to me.

They mark their stainless barrels with "SS 5.56"

Colt will be "CMP" then other info on the bbl... but the 11" w 5.5" FH didn't have any markings on them, I think.

Here are some good resources:

www.biggerhammer.net/ar15/uppers/

and

www.biggerhammer.net/ar15/barrelsbolts/

on Colt markings.

A complete working upper with bolt/carrier is worth prolly around $300 to $350, and more if it was a Colt.

I do not think it is chrome lined, since "SS" tells me it's stainless steel.

Who knows what the twist might be....