I am pretty sure it is an option...of the seven Caravans I have flown only two had it installed. Five had no oxygen gauge, lever, lines, or ports whatsoever. One had the system installed at the factory but it was unservicable prior to being ferried to the operator in order to mount a cockpit voice recorder and FDR to comply with local regulations. For whatever reason that needed to be mounted in the same location as the Cessna oxygen bottle.