I believe that is a Forehand design Iver Johnson Arms & Cycle Works gun. The very few I've seen had a "Modified Diana" monobloc barrel. Dave and John would know.
https://doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=398487&page=all

Or maybe a H&A frown
https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=398670