Pic' 1. I have removed the nipple, and let the back-check/piston fall out the back... I have then re-inserted the correct way around, so you can see the o-ring end protruding from the reverse end of the nipple which is inserted into the bottle first. This is an exaggerated position that it would be in whilst filling the bottle, where a greater pressure of air is forcing it back and diffusing into the bottle...
Pic' 2. I have pushed the back-check/piston in, so it is flush with the real end of the fill nipple. This is the position it would be in when the bottle is holding air; the nipple is in the back and sealing snugly against the side preventing any air escaping.
Pic' 3. I have placed the back-check/piston into the hole where the nipple goes, it is inserted with the o-ring end first... If you do it this way you won't be faffing around bothering whether it's in or not
Pic' 4. I have slapped the remainder of the nipple on top, and will then proceed to screw and tighten it down, adding a small bit of ptfe tape or paste if I choose to...
Everyone clear and happy?