Aluminum tank cap, which is screwed onto the tank (preset) regulator to prevent dirt in the regulator and to protect the thread.
There is also room for 1 or 2 spare O-rings so that you are never without rings if they break.
The set ships for 8 euros [NL] and consists of:
5 Tank O-Rings
1 Tank cap (black aluminum)
*image of tank as example, tank not included.