My third model double-action does not have a through hole like that. I looked at Chicoine's Gunsmithing book, and he has photos of second model single-actions with the end of a screw (or pin) at that location. So at least some of the large frames had something there.
I know there is an internal flat spring that tensions the loading gate and that is held by a screw inside the "hammer area," but don't recall exactly where the screw is. That's the only thing that comes to mind, but I will see if I can look at a couple other Merwins this weekend and perhaps get a better idea.
--DJ