The Best Fittings plug works only in combination with a charging kit or hose.
The standard adapter is qualified for 200bar bottles, but can be extended with the use of this piece.
This way you can use the standard Steyr fill adapter directly in your 300bar dive bottle. ( I know some disagree on this method).
There are also 300 bar versions of the adapter,
but these are for air pistols (extra o ring on top), causing some air waste when decoupling the cilinder.
Best is to have a custom made version of this adapter, without the o ring on top.