There are 3 trim pots on the rear.
1 is time delay (guessing how long sensor must be in a closed state before sounding alarm)
2 is the above mentioned PPM (parts per million?) SENSITIVITY LEVEL?
and 3rd is alarm volume.
There is a barrier strip with 4 contacts if i short 2 of them the alarm sounds.
and lastly a power connection (115V AC)
I am not concerned on how it works or what it does(obvious at this point) but what it was part of.
(Medical, space program, fire monitoring or power generation etc.....)