I have recently purchased a Pre-Ban Colt Sporter "Lightweight" from a local gun shop, it was a consignment gun and is in perfect condition, barely looks like it was fired!!!    

Well, I got a very good price on it, and couldn't wait to get home and "play".  When I field stripped the gun for further inspection, I noted some minor design differences between my "New" pre-ban, and my "Older" post ban Colt H-Bar I have owned for a few years.  Some of which were the design of the bolt carrier, exibiting a full ring at the rear and the width of the rails the bolt carrier exibited.  I also noticed there is some sort of metal block installed in the recieved, above the safety lug, stamped "colt" which is not present in my post ban Colt.

Well then, I loaded a .223 snap cap.  It fed fine, and though the trigger feels a little more "Spongy" than the H-Bar, the action seems to be fine...  Then I ejected the cap, and field stripped the rilfe again...    This time, I was looking for the serial number and happened to notice that the rifle was originally chambered for cal. 7.62 X 39...  All of a sudden, I panicked!!!  Was this actually a Preban lower?  Sure, it featured a preban upper, but since it was a frankenstein gun, it may also be an illegal gun!!!  So I looked up the serial number and rest assured it is a preban number!!!  Whew!  Close!

But now here is my delima.  It is definately a .223 upper. , the snap cap fits the chamber perfectly, so I am comfidant it is .223, but since there is no real indication of who manufactured the upper assembly, other than a few small markings, how do I go about identifying the upper?

Obviously, I could request the name of the seller from the gun shop, but if I do not have to, I do not really want to...  (Unless maybe he still has the original 7.62 X39 upper he may want to sell me cheap!!!)

Anyway, thanks in advance for any help you guys can offer...  I am new to this forum, and did try a search, but it turned up zero results!  

See ya,
JMS

OK, so you know the lower is Colt.  Check the bolt itself, to see if it's marked C MP just brhind the locking lugs.  If so, you've got a Colt bolt in it. The carrier should have a C stamped on the left side.

Next, check the upper for ID marks. There should be a C and either one or two other letters on it, probably on the right lug just ahead of the rear sight.

Finally, look at the barrel, on top, (or maybe on the bottom if it's a really early barrel) and see if it's stamped C MP (C, or B, or "chome bore.)

If it all checks out, it's all Colt.  They did sell some conversion kits to let shooters use the 7.62x39 lowers as .223s by switching back and forth, so you might have on on your piece.