penfold
Well-known member
I will pile on and confirm it's a doofer for undoing (generally drinking/potable but not always)water filter housings; they are not tolerant of overtightening and will break if abused, but then the housings are generally sealed with O rings and do not need to be wound up to a zillion lb/ft of torque so arguably it's intentional.
Possibly, or could be for the air bleed; they vary depending on maker.I suppose the small hexagonal hole is for a drain plug so you can drain the filter before unscrewing it.