Jim
Handled a double quite a few years back which had a bubble spirit level fitted to the rib as a type of sighting aid!!!
That one however was dovetailed to the rib, not screwed.
From memory it was a boxlock Gibbs 450 400 3 1/4".
Others on this site will most likely know that rifle also.

This doesn't help much with your question about the screws in your rib though.