It's a mismatched rifle. The Target Sporter model was sold only with an HBAR barrel. Around $1k would probably be fair.
You may be thinking of the Sporter "Match", or "Competition", both HBars, different twists. These came later starting with the 6600 series.
The rifle in the photos is a Sporter Target (R6551). It had an A2 barrel. That means it is TAPERED under the handguard NOT an HBar. Also NO bayo lug but a 1/7 twist. Certainly has the correct A2 upper receiver for that model.
The only difference there could be in this model, is it could have either an A1, or A2 lower receiver, depending when it was made. Photo the flip side so we can see. There's really no specific date when Colt started using only A2 receivers. Colt basterdized a few thousand rifles in an attempt to use up old stock around the same time (1991).
Because if the GS in the serial number, it may very well be an A1 lower receiver on this one.
The rifle in the photo is NOT an HBar, or it would likely say so on the receiver and it may well be, but it's not a mismatched rifle just by looking at those photos.
What it looks like you have there is an R6551 model (circa 1991) which was designated as a "Sporter" to pacify the AW ban and the A2 Government Model (R6500) which DID have the bayonet lug as the only difference.
The value of the rifle in question could be anywhere between 1000$ to 1400$ unfired with box and everything that came from the factory. Hard to tell just by your photos. An obscure example. I haven't seen many 6551s around.
