peen(n = noun.artifact) - the part of a hammerhead opposite the flat striking surface (may have various shapes);
is a kind of part, portion
is a part of hammerhead

   A round-edged, or hemispherical, end to the head of a hammer or sledge, used to stretch or bend metal by indentation. [1913 Webster]

   To draw, bend, or straighten, as metal, by blows with the peen of a hammer or sledge. [1913 Webster]

peen, n. & v.
--n. the wedge-shaped or thin or curved end of a hammer-head (opp. FACE n. 5a).
--v.tr.
1 hammer with a peen.
2 treat (sheet metal) with a stream of metal shot in order to shape it.

Etymology:
17th c.: also pane, app. f. F panne f. Du. pen f. L pinna point