Top quality product developed for the Apocalypse rebreathers by NA90.  Images courtesy of NA90.

The HUD plugs straight into the O2 Pod, to provide independent 3-cell PPO2 monitoring, simultaneous to the PPO2 displayed on a wrist monitor using the other output from the pod.  Includes hose clip, attaching to ALVBOV breathing hoses.

Each of the sensors corresponds to an LED on the display.  Each LED can show red, green, or orange (both red and green on.)
A button on the box and it controls the HUD: one push turns it on and one push turns it off.
After turn it on and you get orange flashing for 30 seconds that indicates a low battery.
The number of greens is the number of tenths above 1.0 so three green is 1.3 PPO2.
The number of reds is the number of tenths below 1.0 so two red is a PO2 of 0.8