Colt has actually producted 3 Rimfire single actions. All are built on 7/8 size SAA frames. I spend a fair amount of time collecting them:
1) Colt Scout (made 1957 to 1970). Aluminum frame, fixed sights.
2) Colt New Frontier (made 1970 to 1986) Steel frame, adjustable sights
3) Colt Peacemaker (1970 to 1977) Steel frame, fixed sights.
You can find them in .22LR and .22LR/.22WMR convertible. The WMR cylinders are usually unfluted. A variety of barrel lengths were made from 4.75" up to Buntline specials.
These are all avaliable at auction at prices comparable to a new Single Six.
The "nicest" ones I've handled and own are the 1970 to 1977 steel frame models. Their fit and finish are a bit nicer. The 1980's New Frontier's have a much rougher finish and incorporate a strange "lawyer safety" inside the loading gate. (I own one and can post pictures if anyone is interested). The Scouts are nice, but very light due to the alloy frame.
One "bargain" out there: Commemoratives. Many commemoratives were made on the "Scout" model. They did not hold value very well, and as such many have been turned and some fired. If you hunt around, you can find some of these Commemoratives in less than "collector grade condition". They usually sell for a song and smile. I've gotten 2 Commemorative in slightly used condition for $300.