Ruger made fixed sight versions of the Single Six in .22 and in .32 caliber; these are sometimes called "Baby Vaqueros" and they've been out of production for some time. Here's a link to a GunBlast article on the subject:
http://www.gunblast.com/Baby_Vaquero.htm
I've seen them with standard and birdshead grips. There was a Bisley version that I believe was a distributor exclusive.
The .32s were popular in cowboy action, especially with ladies and youth, because the Baby Vaquero satisfied the minimum caliber requirement and the grip/frame size was considerably smaller than the Ruger Vaquero. I still see them for sale used from time to time.