I recently acquired a preban Colt Sporter Rifle with a serial # in the 20,000 range. It has the bayo/hider. The rifle was retired from a state police agency and when I got it there was not a single scratch or wear on the lower. It was configured more or less along the lines of the current model of Colt AR15 A3 Tactical Carbine (I have one of those, too, however, it is LEO/Gov restricted).
What I didn't notice when I purchased the Sporter for my personal use was the mag release side of the lower is smooth, without the raised rounded "well" around the mag release button and other areas that I can only assume were for structural reinforcement (?)
Then it dawned on me that the lower was identical to that on the M16 I use (it's a surplus A1 model). That Colt M16 has had thousands of rounds through it over the years and is still an effective weapon.
So, what difference (other than aesthetics) does this slab-sided lower make in the performance/durability or whatever of an AR15?
Is there any functional difference or difference in internal parts compared to the newer designed lowers?
Also, I assume this is one of the rifles made during the transition from the AR-15 model name to the more PC "Sporter" name. With a 20,000 serial can you determine the approximate date of manufacture?
Thanks for your comments.