I kick myself for not buying one at a Marine Flea Market years back for under $100. Of course I would have never really used it, but it would have gone great with my At-Pac Ferring. A lot more robust than the At-Pac but, you were stuck with the unit, couldn't ditch the shell nor could it accommodate doubles.
Bit of history, if you notice it was originally called the Nautilus, but shortly thereafter it was renamed the "Scubasystem", from what I read, Dacor had already registered then name Nautilus for their hard shell back inflation unit and SP had to change the name.