I have a 1850's Spanish shotgun with virtually the same set trigger mechanism (yes, set trigger on the top barrel) - boy is it slick! I want to make a copy from the one I have, it's just as crisp and sensitive as a double set trigger. The extra horizontal lever allows it to be tuned just so - I'll see if I can post photos in a separate topic. No idea who made yours, looks like a good find.
-Eric