Sitting here fiddling with my MWnN conversion/not a conversion, and I see a screw into the side of the hammer which I somehow never had noticed before. Closer inspection reveals a crescent-shaped toggle in the front face of the hammer just below the firing pin. Manipulation with a toothpick or similar causes it to flip over, the reverse side being convex, preventing the hammer from dropping the last fraction of an inch and impacting the primer. It can easily be disengaged with a finger.
Do other modern conversions have this? Did period conversions? Its practical usefulness seems limited to anyone who actually might have carried a revolver for defensive or nefarious purposes.