Bought an AR upper/barrel yesterday at a gun show.  Seemed like a good buy and I've been looking for a good A1 upper receiver.  I've checked all the reference posts and now need some confirmation and suggestions.

Upper is an A1 with large round FA and a P and M in the right side recess under the sight (Capco/Martin Marietta?).  Receiver had some missing finish and quite a few dings but nothing major.  

Barrel appears to be a new or near new Colt HB marked C MP 5.56 NATO 1/7 HBAR with an 0 near chamber (chrome chamber?).

FSB is marked A, PI? in a circle, and 1.  From the info posts sounds like a late replacement A! unit or M16A2 conversion unit.

Flashider is A1 birdcage unit.  

Paid $175 for the upper so I'm pretty comfortable that I got a decent deal.  

Am I missing something or does this sound like a legit A1 upper that someone fit a later HB to?  

Trying to figure out what to do with this one.  I have a slabside retro rifle and have wanted to build an A1 clone.  Haven't looked in a while, but how hard is it to find the correct lightweight barrel and FSB? Or does someone have a suggestion for the current combo?

Thanks for any comments or suggestions.

You could also build a carbine or XM2 as well. 20" pencil barrels pop up on the EE all the time. I have a funny feeling a couple will about mid week. :) $175 is a great price. You can always sell the barrel and get back most your money.