Not for steam wand tips, but for the top of the wand, where the ball (usually) is.
There are many applications for this o-ring, most of them having to do with various different manufacturers steam wands. For the La Marzocco Linea, these are the larger ones' on the steam valve body that the yoke of the steam wand slips over.
2.62 mm thick with an internal diameter of 10.78 mm.
WARNING: This product can expose you to chemicals including lead, which are known to the State of California to cause cancer, birth defects, or other reproductive harm. For more information, go to www.p65warnings.ca.gov.