^^^ I understand completely.
Your system looks at the lower and then could care less about the upper. Here we get lowers without fanfare and then need to license the installation of the short upper. But the actual parts are available for the price of buying them, it's the buyers responsiblity to not have "one from column A and one from column B" in the same place at the same time until the papers are filled out. You can buy all of the parts you like here without difficulty (other than finding them in the middle of the great gun-buying opanic of 2013".