I remember as a kid, having a long gun with a similar set up to this one...might have even been the same make & model, I don't know. I had mentioned it to a friend of mine several months back, and he was dubious that my memory could be correct, since it's a fairly unusual setup by today's standard.
Thanks for the blast from my past and the proof that I'm not crazy. In a perfect world (or at least a better one), I would make an offer to take her off your hands, since I'm nearly certain that the one I had access to in my youth is no longer accessible to me. Good luck on the sale.