I sold this AR last year and after I sold it I noticed it had M4 engraved on the upper.  The AR was listed as Sporter Target and had a 16" upper with hider and lug.  The guy I bought it from said it was an original.  What gun was this and did I give it away for $1450.  The gun was MINT.  This is the only gun I wish I had never sold.  

Colt Sporter Target was factory equipped with a 20" 1:7 Chrome Lined M16A2 Brl.

Somebody swapped uppers before you got it.

The only thing I see about the gun that looks M4'ish is the upper receiver. There's no M4 profile barrel, no M4 handguards, and no collapsible 4 position buttstock. But it sure is a sweet 16" carbine none-the-less.

That is the worst thing about AR's these days. If you buy used there are so many things on the market that you never know what you're getting. So many kits, etc, it just makes buying used a challenge. I think you did ok with what you got for the carbine however. Maybe not top dollar, but a fair price.

The lower looks like it has a large pivot hole because that sure looks like it has an offset pin to me.  The upper looks like a heavy barrel R6721 upper.  No, this was not an original Colt.

And the detachable sight assy isn't a Colt product at all. Forging mark on the ear is an aftermarket supplying foundry.

And you put a Chinese AK sling on it??