I still don't believe there is such a thing. With so many -200s out there? It's nonsense. you have seen -300 combis, maybe with the window slides down or something. The most I could concede is that they are combis that have temporarily been reconfigured for all-freight ops, but the airline must have been desperate for capacity cos as a freighter a -300 would be less economical than an old -100, simply because of the extra weight and drag from the extended upper deck. It can't really be used for passengers (the ones that pay a fare) cos the only access upstairs on an F is via a retractable ladder to the main deck, not a very dignified entrance. Of course all Fs have seats (usually ten or so 2-2 business class sized ones) up there but it's for airline staff, positioning flight crews, flight mechanics, loadmasters et al.
fly Saha Air 707s daily from Tehran's downtown Mehrabad to Mashhad, Kish Island and Ahwaz