My one-second guess was some kind of mount for a shell catcher, but it looks too complex for something like that. That thing of the left side of pic 2 that looks a little like it has internal threading might be to fit over the hinge of a rear sight but it would render the sight useless.
And why an adjustment provision?
Breaking out of the "scope mount" mindset, could it be for something else... a goomizigah attachment for reloading equipment? Or something from Grandma's egg beater... ?
As the King of Siam said, " 'Tis a puzzlement."