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 #00 2nd smallest of the carriers. 1 size below #0.  Marked with one line and one dot, opposite from the #1.5 size.  A little difficult to tell visually but physically it will fit much tighter and will not be hard to tell the two sizes apart.