I acquired an R&D conversion cylinder for my Pietta 1858. When I first tried to put it in it would not fit. I had to remove a little metal from the base of the frame below where the barrel screws in. Before I removed the metal the face of the cylinder was was just slightly too large to fit. The rear of the cylinder was fine.
Hard to tell if this is your problem without seeing your frame and cylinder. Since you have a revolver that the cylinder does work with why don't you lay them side by side and do some measuring to see what the difference is. I don't think your problem will be with the hand or the spring.
I'm just an amateur that has been working on my own guns for years. Maybe some of the real gun smiths will chime in with their thoughts.
Hope this helps.
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