I'm interested in finding out any information at all (including value) of a Wollensak / Bausch & Lomb triple convertible lens - the first focal length in the series is 18 7/8" . Maximum F-stop at the default configuration is f 7. I don't know what the other lengths are, as the person selling the lens hasn't figured that part out yet. Any help with identifying and valuing the lens would be most appreciated.