High density rubber mast plug for mast tip - 30mm, larger size. Has a hole in the top to accept parallel wall stem type headcap fittings. Conical stem headcaps (e.g. pryde) will NOT fit in this mast plug.
The 30mm plug has a "mushroom cap" with an outer diameter that is just a hair over 30mm. The ribs on the 30mm plug have an outer diameter of about 27mm.