RE: F 161 and fin
That's the "trailing edge or back (rear) edge of the fin.
A quick discussion of fin terminology here:
The "head or root" is the part that fits into the fin box in your board.
The "span" (or length) is that portion of the fin from the tip (the bottom) to the root (where it goes into the board,
The flat surface on th bottom fo the root should be as "fair" with the bottom of your board (adjacent to the fin box) as possible.
The front of the fin, from the front of the foiled section (not the root), to the front of the tip at the other end of the "span" is called the "leading ege".
The "trailing edge" is the back of the fin from the junction with the root to the back of the tip.
The distance around the foil from the leading edge (the front of the blade) to the trailing edge (the rear of the blade) is called chord as it is longer (around the foiled surface) than the straight line distance (through the center of the fin, from front to back.
The leading edge most often has a radius on it and is thicker.
The trailing edge is much thinner and normaly will be two converging straight lines from the highest (thickest) point on foil which will be around 30% back from the leading edge.
Hope this helps,