I just received a Colt Match Target HBAR in a trade and I am trying to determine if it has the correct upper/barrel.  The lower s/n starts with CMH.  It is an A2 model.  The upper is stamped CM on the right rear of the carry handle.  The barrel is 20" and has a bird cage type FS/MB.  The barrel is heavy for about half it's length (forward half) and is stamped C MP 5.56 NATO 1/7.  The lower is a dark finish and the upper is a slightly lighter gray.

It has the small pivot pin (screw type) and takedown pin.  The trigger pins are the larger types.  Can someone also tell me how to identify a block over safety.  I have read that some Colt's have this and it determines availability of replacement triggers.  Another factor is that when I turn the upper upside down and look into the recess at the rear near the takedown stud, the area is milled out in an oval shape - not straight the entire length like some other Colt's I have seen.

Lastly, can someone recommend a good source for a replacement rear sight assembly.  The stock one does not fit very well.  It is loose and sets canted to the left.  The base itself is not a tight fit and the detent spring/pin causes it to set to one side.  I am hoping to find one that I can fit to the handle.  Thanks in advance.

Welcome.  post some photos and folks will be able to help you out better

I will try to answer some of your questions.

It has the small pivot pin (screw type) and takedown pin. The trigger pins are the larger types. Can someone also tell me how to identify a block over safety. I have read that some Colt's have this and it determines availability of replacement triggers

You will know if it has the block. Can't miss it. If the entire space in the receiver from mag well to stock is  open, then it does not have the block. Which is located at the back half of the receiver at the stock end. The block in my Colt actually has "Colt" stamped in it.

Lastly, can someone recommend a good source for a replacement rear sight assembly. The stock one does not fit very well. It is loose and sets canted to the left. The base itself is not a tight fit and the detent spring/pin causes it to set to one side. I am hoping to find one that I can fit to the handle. Thanks in advance.

This is normal. Most rear sights will pivot to some extent. Some more than others. Looking from stock to muzzel, the right side of the sight will rest against the carry handle. Don't even worry about it.

Another factor is that when I turn the upper upside down and look into the recess at the rear near the takedown stud, the area is milled out in an oval shape - not straight the entire length like some other Colt's I have seen.

This part I am not sure about. I will have to pull out the Colt and see if I can see what you are talking about.

Me, my camera and posting pictures are not getting along to well right now.  When I had the old take picture, store on a disk and post from disk, I had no problem.  Wife bought me me of those fancy ones and I still do not have it figuired out.

I was really wondering about the two different finishes - upper vs. lower.  The barrel, should it be a HBAR for the full length?  Concerning the block over - mine does have a bridge sort of.  It is not open totally from the mag well back to the stock - on the lower.  Just behind the safety, there is a small portion that I refer to as a bridge.  In other words, there is a small portion that is not totally milled out, then it goes back open.  BTW, the bridge is alll the way to the top of the lower receiver.  Because of my inabilty to post pictures, can someone point me towards a site with pictures of a block?

Concerning the rear sight, it is off set to the left side more than is acceptable.  The base is the problem.  Concerning the oval milled into the upper.  I found information on www.bighammer.net/ar15/uppers/ that calls this a sear relief cut.  According to that site, Colt did not make the cut I have in AR15's.  They say it was only done on M16's and full auto conversions.  Thanks for the help.