A Swordfish with one extra place for the number, a fin, in a box beside a corner. The number still leaves the Swordfish's columns, but only inside the fin's box.
123456789
123456789
9
6
1
8
5
3
237
237
4
1
236
9
8
5
8
2
5
7
9
1
9
5
8
4
2
6
8
6
3
5
4
5
4
237
6
27
1
8
9
6
9
8
4
1
2
5
3
7
8
2
4
235
23
7
6
9
1
8
The SwordfishThe finLoses a pencil mark
Why only the fin's box
A square can only be cleared if it also sees the fin, because the fin might be the number. That keeps the removals inside the fin's box.
How to spot one
Look for Swordfish that fail by one square. If the stray place shares a box with a corner, you have a finned Swordfish.
What it gives you
Usually one removal. Finned fish turn up on tough boards.
Practise Finned Swordfish
Boards that need it, with the owl's hints when you're stuck.