On another forum, there is a lot of discussion about converting older three-screw Ruger .357s to a different caliber. The gun of choice is the flat-top model made up until around 1963, though the later guns are used also, these still the pre-1973 three screw guns that have the ribs on either side of the rear sight tang.
Why? This vintage of Ruger .357 was built on the smaller frame than current New Model .357s, and is closer in size to the old Colt Single Action. I've had one recently converted to .44 Special, and several posted photos of both .44 Specials and .38-40 caliber. There have been some converted to .45 Colt.
There is also available a two-piece grip frame that duplicates the Colt, and allows use of one-piece style grips. So, a conversion gives the shooter a near duplicate of a Colt, in a target sighted modern revolver with coil springs and frame mounted firiing pin. Case colored by Doug Turnbull, the result is about the epitome of the single action revolver.