Here's my small contribution. On top is my 66-1 with 6 1/8 barrel, and below it is my 66-3 with a barrel that measures exactly 6". I hope you can see the difference, as my photography is not the best.
I remember reading somewhere (can't remember where or when) that a lot of the barrel lengths were 5 7/8" back in the late 70's. They were advertised as 6", and some owners were getting upset and threatened a class action suit against S&W. So the factory made the barrels 6 1/8" for a short while. Fact or fiction?, I don't know.
I lettered the 66-1 and mentioned that the barrel was 6 1/8", but the letter only said that it was shipped with a 6" barrel. No mention of the extra 1/8".