Colt used a variety of differences (mentioned in this thread) to try to prevent interchangeability between its civilian AR's and the military M16's. In the early 1980's, Colt's patents ran out and several competitors entered the market. These competitors made guns that were much closer to the military standards, with the obvious exception of the "third hole" for the auto sear. Since the ATF eventually came around to agreeing that the "third hole" was the key determinant of whether a gun was a machine gun, or not, the competitors' variants became the industry standard. Finally Colt yielded and made their guns like everybody else's.
A "large pin" conversion to full automatic (which could have been done prior to May 19, 1986), is less desirable than a "small pin" conversion. (Referring to registered receivers.) This should be reflected in the price.