I think its Dad must be a sailor.

Or maybe a "Navel" rating.
Recent citrus addition to the Sainsbury stable. I think they were Tarran Tarra-ing them as new? different? better? or some such.
For our palates they're a wee bit tart, but quite novel in that the small, inner one is both larger than the traditional "Navel" and, actually, edible as a unit in its own right.