Pietta 5-round R&D New Army conversion cylinder
As near as I can tell, there is also an extra cylinder notch that probably allows you to release the hammer at half-cock into the safety notch ground into the backplate and then the bolt fits in that extra notch on the cylinder as well, they match up I'm guessing.
But that might be a trick of the camera angle as I'm looking at the cylinder notch closest to the background, the rightmost cylinder notch as we look at the picture.
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