When I bought my slide, I didn't know the value. Mine needed a lot of work, and the slide handle ring was missing, so the one up for sale now is worth much more.
The National Music Museum in South Dakota once claimed that their Reiffel & Husted slide saxophone was "the only one that has survived" but since I personally know of five of them, even museums can be wrong.
I did get the slide sax working well, and I plan a video, hopefully soon. The biggest advantage of the Reiffel & Husted slide sax is its all metal construction. There is no rubber gasket to leak to deteriorate.
There are at least 4 or 5 varieties of slide saxes. One even has keys operated by a slide. The Paul Cohen slide sax article is good (Saxophone Journal, July/August & September/October 1994). What issue has the 3rd article, Pete?