Early first year guns have a SS rear sight, which was replaced due to complaints - I was told it was a visual problem for people, esp LE. I own one of those.
The 66 was available in 2.5, 3, 4, and 6 in barrel lengths. There may have been a 5in barrel, special order and I never saw one with an 8 3/8 barrel myself, but they could exist. As another has said, round or square butt was another choice. Around 1980 recessed chambers were removed, and not long after that, the pinned barrels as well.
Many of the changes from a 66, to a 66-1, to a 66-2, were minute, but the factory gave it a model designation change. Many of the changes had to do with production, or internals, and not things like pinned and recessed or not pinned and recessed, which never designated a new dash number. Other things like hammer, trigger options, grip types and sight options never designated a number change either.