I typically used this anytime over 8,000'.  It is gently used in excellent condition.  It is the Aerox D Cylinder which holds 13 cubic feet of oxygen and it has two hook up ports.  Comes with one unused inflatable face mask and a finger oxygen sensor so you know where to set the system This system goes for $477 new at ACS: https://www.aircraftspruce.com/catalog/pspages/aeroxsys.php?clickke...

The only thing you need to add are individual cannulas with flow meter and fitting which go for $120 each: https://www.aerox.com/4110-705-oxysaver-cannula-kit-with-needle-val...

It's a great system and it's longest use was a cross country flight from west coast to east coast at 15,500' and it performed flawlessly for two of us.

My price to you is $75 plus shipping.  So you end up getting the $477 system for $315.

Email is: donmorrisey at gmail dot com

Text is: 617- eight two eight - 5550