I'd lean towards it being a sight, too. I think if it was for some sort of lanyard the rear of the top lever would have just been pierced from side to side, creating a hole, instead of creating a flip up jobbie.
'Tis true that an aperture sharpens the focus, but it is usually in the range of the front sight, not way out at the target, and especially so for an aperture this large. The smaller the aperture the greater the depth of field in clarity. Here's a good article on how this works, and the parallax suppression effect of an aperture sight. The effect of making the front bead clearer would only work
against good shotgunning, because the focus should never be on the bead, or muzzle, but on the bird.
Strange deal on a break open gun, but as has been said here many times before, in other contexts, one could have just about any contraption one wanted on a gun if willing to pay the price.
http://dougkerr.net/Pumpkin/articles/Aperture_Sight.pdf SRH
