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Not sure if this is for an original or reproduction, but it's a start. It's from Dixie Gun Works in Union City, Tennessee.
I had successfully crafted one for my original 1863 Starr single action. The flat arm to the left rides on a little nob near the pivot point of the hammer. I hope this information helps.
In passing by this thread, I to have posted information here near 10 years ago.
Regards.