End Piece
Yep, it's just the piece at the end
The "end piece," or "end graft" is a decorative wedge or other design element at the point where the sides of an instrument join. This is the classic Martin end piece, essentially unchanged in style for about 150 years:
Some makers use the same material for the end piece that they use for binding, to give a unified look. Others treat the end piece as a unique piece of art.