If the problem is a block/nut which was part of the scope mount system then a schematic of the rear sight isn't going to do you any good as it has nothing to do with the factory sight.

The rear sight elevation nut is retained in a "T" slot, which is what the nut/block you are having trouble with is. There isn't anything holding that piece in the slot unless it was an oversize part which was driver into the slot from the rear, or the prior owner used either a Loc-Tite product or Epoxy to glue the block in. In either case that piece should be relatively easy to drive backward out if the slot. Just be careful not to damage the frame.