High density rubber mast plug for mast tip - 28mm, smaller 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 28mm plug has a "mushroom cap" with an outer diameter that is just a hair over 28mm. The ribs on the 28mm plug have an outer diameter of about 25mm.