Carrier for the Power Tube O-ring. There are a bunch of these to fit every variation in O-ring size. O-ring included. You can also buy the O-rings separately.  New Size #000 smallest of the carriers. 2 sizes below #0.  Marked with just one line, opposite side from the #1 size.  A little difficult to tell visually but physically it will fit super tight and will not be hard to tell.