I bought the R&D drop-in from Midway for my 58 Pietta. It actually dropped right in and works fine.
It's a good idea not to use loads that exceed R&D's recommendations in it though (stick to cowboy loads). I just got done loading 300 rounds for my upcoming camping trip in June. I loaded half the cartridges with 4.9gr Trail Boss and the other half with 5.4gr. With the 250gr LRNFP bullets, these should all go between 600 and 700 fps. Nice and mild and fun to shoot.