Search, then "3" model 66" using Google Custom Search, is your friend - you can probably stop short of reading all 14,500 posts
:
http://smith-wessonforum.com/searchresults.php?siteurl=smith-wessonforum.com%2Fs-w-revolvers-1961-1980%2F332218-identify-your-gun.html&ref=&ss=2641j960007j12&cx=partner-pub-2376253589436827%3A2526757862&cof=FORID%3A10&ie=UTF-8&q=3"+model+66&sa=Go
And here is one with your specific question answered:
History of the 3" Model 66
Hope this is helpful.