After attempting to find this exact complete mechanism, including counterweight, in various patents etc. I've come to the conclusion that the most likely ID of the maker/inventor is Mr. Mann. One of his patents shows a breech mechanism with screw-press closure like this model. It also includes breechblock supports floating in journals at the end of each side of the strap connecting the breech to the trunnions. Other details differ but Mann is the only inventor I've found who used both features in one design of the class of breechloaders we've come to describe as "strap guns."